This came up because the USM implementation with -foffload-memory={unified,pinned}
as posted at https://gcc.gnu.org/pipermail/gcc-patches/2022-July/597976.html
does not handle USM with static variables.

This shows up for the OG12 alias devel/omp/gcc-12 branch as FAIL for requires-4.c.

The attached patch prepares for skipping requires-4.c for the gcn/nvptx device
and adds an adjacent requires-4a.c testcase, using heap memory, that can still
run on gcn/nvptx.

Additionally, I commented on no longer used #defined, following the
precedence GOMP_DEVICE_HOST_NONSHM.

Thus, this tests adds another testcase and one effective-target check,
out-comments a unused #define - and that's it.
(Otherwise, it is just a prep patch.)

OK for mainline?

Tobias

PS: Currently, neither the preexisting offload_device_nvptx nor the new
offload_device_gcn target selector is used, neither in old code nor by this patch.
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