gnu: cross-base: Don't specify mingw --with-newlib

Previous to this commit, we added a --with-newlib configure flag to
cross-gcc when cross-newlib?, but cross-newlib? is true only when
target-mingw?.

It turns out that specifying --with-newlib disables the
GLIBCXX_CROSSCONFIG check, which is used to detect
_GLIBCXX_HAVE__WFOPEN, which is required in C++17 std::filesystem for
mingw-w64 systems.

Additional context: In gnu/packages/embedded.scm, --with-newlib is
specified explicitly when we're actually using newlib, which seems like
the correct way of handling it.

Situation in other distros:
- Debian's gcc-mingw-w64 doesn't specify --with-newlib
- Fedora's mingw64-gcc-c++ explicitly specifies --without-newlib

Chesterton's fence: Chatting with janneke, who originally added this
mechanism, reveals that this flag is not only no longer required, but
also that removing it doesn't break his guile-mingw builds. See IRC logs
of #guix for 2022-02-15.

* gnu/packages/cross-base.scm (cross-gcc-arguments): Don't check for and
specify --with-newlib.
(cross-libc): Check for mingw and use mingw-w64 directly.
(cross-newlib?): Remove, unexport.
(native-libc): Remove.

gnu/packages/cross-base.scm

@@ -42,7 +42,6 @@ (define-module (gnu packages cross-base)
   #:export (cross-binutils
             cross-libc
             cross-gcc
-            cross-newlib?
             cross-kernel-headers))
 
 (define-syntax %xgcc
@@ -177,10 +176,6 @@ (define (cross-gcc-arguments target xgcc libc)
                              `((string-append "--with-toolexeclibdir="
                                               (assoc-ref %outputs "lib")
                                               "/" ,target "/lib"))
-                             '())
-                       ;; For a newlib (non-glibc) target
-                       ,@(if (cross-newlib? target)
-                             '("--with-newlib")
                              '()))
 
                  ,(if libc
@@ -489,10 +484,11 @@ (define* (cross-libc target
                      (xheaders (cross-kernel-headers target)))
   "Return LIBC cross-built for TARGET, a GNU triplet. Use XGCC and XBINUTILS
 and the cross tool chain."
-  (if (cross-newlib? target libc)
-      (native-libc target libc
-                   #:xgcc xgcc
-                   #:xbinutils xbinutils)
+  (if (target-mingw? target)
+      (let ((machine (substring target 0 (string-index target #\-))))
+        (make-mingw-w64 machine
+                        #:xgcc xgcc
+                        #:xbinutils xbinutils))
       (package
         (inherit libc)
         (name (string-append "glibc-cross-" target))
@@ -553,24 +549,6 @@ (define* (cross-libc target
                          ,@(package-inputs libc)  ;FIXME: static-bash
                          ,@(package-native-inputs libc))))))
 
-(define* (native-libc target
-                     #:optional
-                     (libc glibc)
-                     #:key
-                     xgcc
-                     xbinutils)
-  (if (target-mingw? target)
-      (let ((machine (substring target 0 (string-index target #\-))))
-        (make-mingw-w64 machine
-                        #:xgcc xgcc
-                        #:xbinutils xbinutils))
-      libc))
-
-(define* (cross-newlib? target
-                       #:optional
-                       (libc glibc))
-  (not (eq? (native-libc target libc) libc)))
-
 
 ;;; Concrete cross tool chains are instantiated like this:
 ;;