This patch handles Unified Shared Memory (USM) in the OpenMP runtime routine
omp_target_is_accessible implementation.

A previous patch was submitted some months ago
(https://gcc.gnu.org/pipermail/gcc-patches/2022-May/594187.html) but not yet
reviewed due to dependencies on the Unified Shared Memory implementation.
Although USM is not yet in mainline, the corresponding patches were already
committed to OG12. I rebased, updated, and committed my patch to OG12
(devel/omp/gcc-12 branch).

I tested the patch with nvptx offloading (x86_64-linux and PowerPC) without
regressions. Since USM is not supported for all gcn targets, I tested gcn with
offloading for x86_64-linux on various targets (gfx90a, gfx908, gfx906, gfx803)
- also without regressions.

Marcel
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