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

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.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D115144

Author: JonChesterfield

openmp/libomptarget/test/lit.cfg

@@ -78,6 +78,8 @@ if config.libomptarget_current_target.startswith('nvptx'):
   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')