cray: detect shasta os properly (#17467)

Fixes #17299

Cray Shasta systems appear to use an unmodified Sles or other Linux operating system on the backend (like Cray "Cluster" systems and unlike Cray "XC40" systems that use CNL).

This updates the CNL version detection to properly note that this is the underlying OS instead of CNL and delegate to LinuxDistro.

lib/spack/spack/operating_systems/cray_backend.py

@@ -97,6 +97,9 @@ def __str__(self):
     def _detect_crayos_version(cls):
         if os.path.isfile(_cle_release_file):
             release_attrs = read_cle_release_file()
+            if 'RELEASE' not in release_attrs:
+                # This Cray system uses a base OS not CLE/CNL
+                return None
             v = spack.version.Version(release_attrs['RELEASE'])
             return v[0]
         elif os.path.isfile(_clerelease_file):

lib/spack/spack/platforms/cray.py

@@ -20,7 +20,7 @@
 _craype_name_to_target_name = {
     'x86-cascadelake': 'cascadelake',
     'x86-naples': 'zen',
-    'x86-rome': 'zen',  # Cheating because we have the wrong modules on rzcrayz
+    'x86-rome': 'zen2',
     'x86-skylake': 'skylake_avx512',
     'mic-knl': 'mic_knl',
     'interlagos': 'bulldozer',