[openmp][amdgpu] Disable tests requiring USM on amdgcn

[JonChesterfield]

Summary

These tests tend to hang or crash on hardware that doesn't
support USM. Disabling them helps diagnose other issues. To safely
enable we require a means of testing whether USM is expected to work.

[jdoerfert]

LG

openmp/libomptarget/test/lit.cfg

 if config.libomptarget_current_target.startswith('nvptx'):
   try:
     cuda_arch = int(config.cuda_test_arch)
     if cuda_arch < 70:
       supports_unified_shared_memory = False
   except ValueError:
     # If the architecture is invalid, assume it is supported.
     supports_unified_shared_memory = True
+if config.libomptarget_current_target.startswith('amdgcn'):
+    supports_unified_shared_memory = False
 if supports_unified_shared_memory:
    config.available_features.add('unified_shared_memory')
 
 # Setup environment to find dynamic library at runtime
 if config.operating_system == 'Windows':
     append_dynamic_library_path('PATH', config.library_dir, ";")
     append_dynamic_library_path('PATH', config.omp_host_rtl_directory, ";")
 elif config.operating_system == 'Darwin':