[OpenMP][libomptarget] Enable automatic unified shared memory executi… (#75999)

…on (zero-copy) on MI300A.

This patch enables applications that did not request OpenMP
unified_shared_memory to run with the same zero-copy behavior, where
mapped memory does not result in extra memory allocations and memory
copies, but CPU-allocated memory is accessed from the device. The name
for this behavior is "automatic zero-copy" and it relies on detecting:
that the runtime is running on a MI300A, that the user did not select
unified_shared_memory in their program, and that XNACK (unified memory
support) is enabled in the current GPU configuration. If all these
conditions are met, then automatic zero-copy is triggered.

This patch is still missing support for global variables, which will be
provided in a subsequent patch.

Co-authored-by: Thorsten Blass <thorsten.blass@amd.com>