[llvm] Recognize arm64 as target-aarch64 in LIT

[JDevlieghere]

Summary

Recognize arm64 triples as AArch64 so we can XFAIL/skip tests using target-aarch64.

[jhenderson]

Looks reasonable to me. I guess I'd be somewhat curious if this changed the number of tests that are now being run.

[JDevlieghere]

In D119169#3303585, @jhenderson wrote:

Looks reasonable to me. I guess I'd be somewhat curious if this changed the number of tests that are now being run.

No difference on Darwin with and without the patch:

Testing Time: 244.61s

Unsupported      :   667
Passed           : 46288
Expectedly Failed:   168

Testing Time: 263.27s

Unsupported      :   667
Passed           : 46288
Expectedly Failed:   168

llvm/utils/lit/lit/llvm/config.py

def __init__(self, lit_config, config):
             if re.match(r'^x86_64.*-linux', target_triple):
                 features.add('x86_64-linux')
             if re.match(r'^i.86.*', target_triple):
                 features.add('target-x86')
             elif re.match(r'^x86_64.*', target_triple):
                 features.add('target-x86_64')
             elif re.match(r'^aarch64.*', target_triple):
                 features.add('target-aarch64')
+            elif re.match(r'^arm64.*', target_triple):
+                features.add('target-aarch64')
             elif re.match(r'^arm.*', target_triple):
                 features.add('target-arm')
 
         use_gmalloc = lit_config.params.get('use_gmalloc', None)
         if lit.util.pythonize_bool(use_gmalloc):
             # Allow use of an explicit path for gmalloc library.
             # Will default to '/usr/lib/libgmalloc.dylib' if not set.
             gmalloc_path_str = lit_config.params.get(