spack arch: add --family --generic flags (#47078)

This allows users to do:

```
spack install ... target=$(spack arch --target --family)
spack install ... arch=$(spack arch --family)

spack install ... target=$(spack arch --target --generic)
spack install ... arch=$(spack arch --generic)
```

Deprecate `--generic-target` in favor of `--generic --target`

lib/spack/spack/cmd/arch.py

@@ -19,12 +19,23 @@
 
 
 def setup_parser(subparser):
+    # DEPRECATED: equivalent to --generic --target
     subparser.add_argument(
-        "-g", "--generic-target", action="store_true", help="show the best generic target"
+        "-g",
+        "--generic-target",
+        action="store_true",
+        help="show the best generic target (deprecated)",
     )
     subparser.add_argument(
         "--known-targets", action="store_true", help="show a list of all known targets and exit"
     )
+    target_type = subparser.add_mutually_exclusive_group()
+    target_type.add_argument(
+        "--family", action="store_true", help="print generic ISA (x86_64, aarch64, ppc64le, ...)"
+    )
+    target_type.add_argument(
+        "--generic", action="store_true", help="print feature level (x86_64_v3, armv8.4a, ...)"
+    )
     parts = subparser.add_mutually_exclusive_group()
     parts2 = subparser.add_mutually_exclusive_group()
     parts.add_argument(
@@ -80,6 +91,7 @@ def display_target_group(header, target_group):
 
 def arch(parser, args):
     if args.generic_target:
+        # TODO: add deprecation warning in 0.24
         print(archspec.cpu.host().generic)
         return
 
@@ -96,6 +108,10 @@ def arch(parser, args):
     host_platform = spack.platforms.host()
     host_os = host_platform.operating_system(os_args)
     host_target = host_platform.target(target_args)
+    if args.family:
+        host_target = host_target.family
+    elif args.generic:
+        host_target = host_target.generic
     architecture = spack.spec.ArchSpec((str(host_platform), str(host_os), str(host_target)))
 
     if args.platform: