spack arch: add --generic argument (#27061)

The `--generic` argument allows printing the best generic target for the
current machine. This can be quite handy when wanting to find the
generic architecture to use when building a shared software stack for
multiple machines.

lib/spack/spack/cmd/arch.py

@@ -20,6 +20,10 @@
 
 
 def setup_parser(subparser):
+    subparser.add_argument(
+        '-g', '--generic-target', action='store_true',
+        help='show the best generic target'
+    )
     subparser.add_argument(
         '--known-targets', action='store_true',
         help='show a list of all known targets and exit'
@@ -74,6 +78,10 @@ def display_target_group(header, target_group):
 
 
 def arch(parser, args):
+    if args.generic_target:
+        print(archspec.cpu.host().generic)
+        return
+
     if args.known_targets:
         display_targets(archspec.cpu.TARGETS)
         return