[lit][AIX] Add LIBPATH to pass through env vars

[daltenty]

Summary

AIX uses LIBPATH to specify the library search path in addition to LD_LIBRARY_PATH, and a lot of users / tooling will use it preferentially. In lit we currently pass through LD_LIBRARY_PATH but not LIBPATH in the env on AIX, this patch corrects this inconsistency.

[hubert.reinterpretcast]

LGTM; thanks!

llvm/utils/lit/lit/TestingConfig.py

def fromdefaults(litConfig):
             'VSINSTALLDIR',
             'WindowsSdkDir',
             'WindowsSDKLibVersion',
             'SOURCE_DATE_EPOCH',
             'GTEST_FILTER',
             'DFLTCC',
         ]
 
-        if sys.platform == 'win32':
+        if sys.platform.startswith('aix'):
+            pass_vars += ['LIBPATH']
+        elif sys.platform == 'win32':
             pass_vars += [
                 'COMSPEC',
                 'INCLUDE',
                 'LIB',
                 'PATHEXT',
                 'USERPROFILE',
             ]
             environment['PYTHONBUFFERED'] = '1'