gnu: GCC: Switch to GCC 10.

* gnu/packages/patches/gcc-cross-gxx-include-dir.patch: New file.
* gnu/packages/patches/gcc-10-cross-environment-variables.patch: New file.
* gnu/packages/gcc.scm (gcc): Point to GCC-10.
(gcc-objc): Point to GCC-OBJC-10.
(gcc-objc++): Point to GCC-OBJC++-10.
* gnu/packages/cross-base.scm (cross-gcc)[source](patches): Apply the new
patches.

gnu/packages/cross-base.scm

@@ -243,6 +243,9 @@ (define* (cross-gcc target
         (append
          (origin-patches (package-source xgcc))
          (append (cond
+                  ((version>=? (package-version xgcc) "10.0")
+                   (search-patches "gcc-10-cross-environment-variables.patch"
+                                   "gcc-cross-gxx-include-dir.patch"))
                   ((version>=? (package-version xgcc) "8.0")
                    (search-patches "gcc-8-cross-environment-variables.patch"))
                   ((version>=? (package-version xgcc) "6.0")

gnu/packages/gcc.scm

@@ -598,7 +598,7 @@ (define-public gcc-11
 
 ;; Note: When changing the default gcc version, update
 ;;       the gcc-toolchain-* definitions.
-(define-public gcc gcc-8)
+(define-public gcc gcc-10)
 
 (define-public (make-libstdc++ gcc)
   "Return a libstdc++ package based on GCC.  The primary use case is when
@@ -838,7 +838,7 @@ (define-public gcc-objc-10
   (custom-gcc gcc-10 "gcc-objc" '("objc")
               %objc-search-paths))
 
-(define-public gcc-objc gcc-objc-7)
+(define-public gcc-objc gcc-objc-10)
 
 (define %objc++-search-paths
   (list (search-path-specification
@@ -880,7 +880,7 @@ (define-public gcc-objc++-10
   (custom-gcc gcc-10 "gcc-objc++" '("obj-c++")
               %objc++-search-paths))
 
-(define-public gcc-objc++ gcc-objc++-7)
+(define-public gcc-objc++ gcc-objc++-10)
 
 (define (make-libstdc++-doc gcc)
   "Return a package with the libstdc++ documentation for GCC."

gnu/packages/patches/gcc-10-cross-environment-variables.patch

@@ -0,0 +1,51 @@
+Search path environment variables for cross-compilers.  See the discussion
+at <http://gcc.gnu.org/ml/gcc/2013-02/msg00124.html>.
+
+Note: Touch 'C_INCLUDE_PATH' et al. rather than 'CPATH', as discussed
+at <http://bugs.gnu.org/22186>.
+
+diff --git a/gcc/gcc.c b/gcc/gcc.c
+--- a/gcc/gcc.c
++++ b/gcc/gcc.c
+@@ -4474,7 +4474,7 @@ process_command (unsigned int decoded_options_count,
+     }
+ 
+   temp = env.get (LIBRARY_PATH_ENV);
+-  if (temp && *cross_compile == '0')
++  if (temp)
+     {
+       const char *startp, *endp;
+       char *nstore = (char *) alloca (strlen (temp) + 3);
+diff --git a/gcc/incpath.c b/gcc/incpath.c
+--- a/gcc/incpath.c
++++ b/gcc/incpath.c
+@@ -472,8 +472,8 @@ register_include_chains (cpp_reader *pfile, const char *sysroot,
+ 			 int stdinc, int cxx_stdinc, int verbose)
+ {
+   static const char *const lang_env_vars[] =
+-    { "C_INCLUDE_PATH", "CPLUS_INCLUDE_PATH",
+-      "OBJC_INCLUDE_PATH", "OBJCPLUS_INCLUDE_PATH" };
++    { "CROSS_C_INCLUDE_PATH", "CROSS_CPLUS_INCLUDE_PATH",
++      "CROSS_OBJC_INCLUDE_PATH", "CROSS_OBJCPLUS_INCLUDE_PATH" };
+   cpp_options *cpp_opts = cpp_get_options (pfile);
+   size_t idx = (cpp_opts->objc ? 2: 0);
+ 
+@@ -484,7 +484,7 @@ register_include_chains (cpp_reader *pfile, const char *sysroot,
+ 
+   /* CPATH and language-dependent environment variables may add to the
+      include chain.  */
+-  add_env_var_paths ("CPATH", INC_BRACKET);
++  add_env_var_paths ("CROSS_CPATH", INC_BRACKET);
+   add_env_var_paths (lang_env_vars[idx], INC_SYSTEM);
+ 
+   target_c_incpath.extra_pre_includes (sysroot, iprefix, stdinc);
+diff --git a/gcc/system.h b/gcc/system.h
+--- a/gcc/system.h
++++ b/gcc/system.h
+@@ -1244,4 +1244,6 @@ void gcc_stablesort (void *, size_t, size_t,
+    of the number.  */
+ #define PRsa(n) "%" #n PRIu64 "%c"
+ 
++#define LIBRARY_PATH_ENV "CROSS_LIBRARY_PATH"
++
+ #endif /* ! GCC_SYSTEM_H */

gnu/packages/patches/gcc-cross-gxx-include-dir.patch

@@ -0,0 +1,73 @@
+This patch reverts upstream commit b4d3485e4fc1d:
+
+  https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=b4d3485e4fc1d029e620a59deb54b3f4f3f6b209
+
+Otherwise, GCC ends up searching the wrong target directory for cross-headers.
+
+diff --git a/gcc/configure b/gcc/configure
+--- a/gcc/configure
++++ b/gcc/configure
+@@ -3689,11 +3689,6 @@ esac
+ fi
+ 
+ 
+-# If both --with-sysroot and --with-gxx-include-dir are passed, we interpolate
+-# the former in the latter and, upon success, compute gcc_gxx_include_dir as
+-# relative to the sysroot.
+-gcc_gxx_include_dir_add_sysroot=0
+-
+ # This logic must match libstdc++-v3/acinclude.m4:GLIBCXX_EXPORT_INSTALL_INFO.
+ if test x${gcc_gxx_include_dir} = x; then
+   if test x${enable_version_specific_runtime_libs} = xyes; then
+@@ -3705,10 +3700,15 @@ if test x${gcc_gxx_include_dir} = x; then
+     fi
+     gcc_gxx_include_dir="\$(libsubdir)/\$(libsubdir_to_prefix)$libstdcxx_incdir"
+   fi
+-elif test "${with_sysroot+set}" = set; then
++fi
++
++gcc_gxx_include_dir_add_sysroot=0
++if test "${with_sysroot+set}" = set; then
+   gcc_gxx_without_sysroot=`expr "${gcc_gxx_include_dir}" : "${with_sysroot}"'\(.*\)'`
+   if test "${gcc_gxx_without_sysroot}"; then
+-    gcc_gxx_include_dir="${gcc_gxx_without_sysroot}"
++    if test x${with_sysroot} != x/; then
++      gcc_gxx_include_dir="${gcc_gxx_without_sysroot}"
++    fi
+     gcc_gxx_include_dir_add_sysroot=1
+   fi
+ fi
+diff --git a/gcc/configure.ac b/gcc/configure.ac
+index 715fcba0482..88136e232df 100644
+--- a/gcc/configure.ac
++++ b/gcc/configure.ac
+@@ -204,11 +204,6 @@ no)	;;
+ *)	gcc_gxx_include_dir=$with_gxx_include_dir ;;
+ esac])
+ 
+-# If both --with-sysroot and --with-gxx-include-dir are passed, we interpolate
+-# the former in the latter and, upon success, compute gcc_gxx_include_dir as
+-# relative to the sysroot.
+-gcc_gxx_include_dir_add_sysroot=0
+-
+ # This logic must match libstdc++-v3/acinclude.m4:GLIBCXX_EXPORT_INSTALL_INFO.
+ if test x${gcc_gxx_include_dir} = x; then
+   if test x${enable_version_specific_runtime_libs} = xyes; then
+@@ -220,10 +215,15 @@ if test x${gcc_gxx_include_dir} = x; then
+     fi
+     gcc_gxx_include_dir="\$(libsubdir)/\$(libsubdir_to_prefix)$libstdcxx_incdir"
+   fi
+-elif test "${with_sysroot+set}" = set; then
++fi
++
++gcc_gxx_include_dir_add_sysroot=0
++if test "${with_sysroot+set}" = set; then
+   gcc_gxx_without_sysroot=`expr "${gcc_gxx_include_dir}" : "${with_sysroot}"'\(.*\)'`
+   if test "${gcc_gxx_without_sysroot}"; then
+-    gcc_gxx_include_dir="${gcc_gxx_without_sysroot}"
++    if test x${with_sysroot} != x/; then
++      gcc_gxx_include_dir="${gcc_gxx_without_sysroot}"
++    fi
+     gcc_gxx_include_dir_add_sysroot=1
+   fi
+ fi