[OpenMP] Fix two usm tests for amdgpus. (#78824)

Some are missing setting of HSA_XNACK=1 environment variable, used to
enable unified memory support on amdgpu's when it's not been set at
kernel boot time. Some others needed to be marked as supporting
unified_shared_memory in the lit test harness.

Extend lit test harness to enable unified_shared_memory requirement for
AMD GPUs.

Reland: #77851