GCC: Conflict for <12 for M1 (#26318)

aarch64/M1 is only a supported build combination for GCC in
the planned GCC 12+ release.

var/spack/repos/builtin/packages/gcc/package.py

@@ -225,6 +225,10 @@ class Gcc(AutotoolsPackage, GNUMirrorPackage):
     # Binutils can't build ld on macOS
     conflicts('+binutils', when='platform=darwin')
 
+    # aarch64/M1 is supported in GCC 12+
+    conflicts('@:11.99', when='target=aarch64: platform=darwin',
+              msg='Only GCC 12 and newer support macOS M1 (aarch64)')
+
     # Newer binutils than RHEL's is required to run `as` on some instructions
     # generated by new GCC (see https://github.com/spack/spack/issues/12235)
     conflicts('~binutils', when='@7: os=rhel6',