Mark GCC7 and newer as conflicting with rhel6 without binutils (#14706)

When building gcc7 and gcc8 on RHEL6 with Spack and installing it as
a spack-available compiler, OpenBLAS will fail to compile because GCC
generates newer instructions than rhel6's `as` assembler knows about
(e.g. "vpermpd").

Building gcc8 with binutils succeeds, and it generates a GCC that can
then successfully build OpenBLAS. This is also expected to work for
gcc7 on RHEL6.

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

@@ -193,6 +193,11 @@ class Gcc(AutotoolsPackage, GNUMirrorPackage):
     # Binutils can't build ld on macOS
     conflicts('+binutils', when='platform=darwin')
 
+    # 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',
+              msg='New GCC cannot use system assembler on RHEL6')
+
     if sys.platform == 'darwin':
         # Fix parallel build on APFS filesystem
         # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81797