[Libomptarget] Make unified shared memory test unsupported on AMDGPU

This test is an expected failure on AMDGPU. The expected failure is a GPU memory
failure, which will typically result in the device totally failing. This isn't
an issue for some GPU configurations that do not use the offloading device to
also drive the display server. However, if the main GPU is used for testing it
will reliably result in the user's display becoming unresponsive. This makes it
difficult to run the GPU offloading tests on many systems.

This patch simply makes this test unsupported so it no longer runs and freezes
my computer when using `ninja check-openmp`.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D132891