[OpenMP] Fix two usm tests for amdgpus. (#77851)

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.

GitOrigin-RevId: 3add9491cd748cb9d71ff38f1517cf06b9708d01
4 files changed