gnu: chez-scheme-for-system: Adjust for bytecode backend.

Change 'chez-scheme-for-system' to return 'chez-scheme-for-racket' in
two cases when it previously returned 'chez-scheme':

 1. When 'chez-scheme' is not distributed with bootstrap bootfiles,
    since we can only cross-compile 'chez-scheme' for those systems; and

 2. When neither 'chez-scheme' nor 'chez-scheme-for-racket' supports
    native code generation, since we will be able to use the ``portable
    bytecode'' backends of 'chez-scheme-for-racket' as soon as the
    issues in <https://racket.discourse.group/t/950> are resolved.

* gnu/packages/chez.scm (chez-scheme-for-racket): Change condition.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/packages/chez.scm

@@ -69,14 +69,17 @@ (define-module (gnu packages chez)
 (define* (chez-scheme-for-system #:optional
                                  (system (or (%current-target-system)
                                              (%current-system))))
-  "Return 'chez-scheme' unless only 'chez-scheme-for-racket' supports SYSTEM,
-including support for native threads."
-  (if (or
-       ;; full support upstream
-       (and=> (chez-upstream-features-for-system system)
-              (cut memq 'threads <>))
-       ;; no support anywhere
-       (not (nix-system->chez-machine system)))
+  "Return 'chez-scheme' if it fully supports SYSTEM, including support for
+bootstrapping and native threads.  Otherwise, return
+'chez-scheme-for-racket'."
+  (if (and=> (chez-upstream-features-for-system system)
+             (lambda (features)
+               (every (cut memq <> features)
+                      '(threads
+                        ;; We can cross-compile for platforms without
+                        ;; bootstrap bootfiles, but we can't self-host
+                        ;; on them short of adding more binary seeds.
+                        bootstrap-bootfiles))))
       chez-scheme
       chez-scheme-for-racket))