libgomp: Enable USM for AMD APUs and MI200 devices

libgomp: Enable USM for AMD APUs and MI200 devices

If HSA_AMD_SYSTEM_INFO_SVM_ACCESSIBLE_BY_DEFAULT is true,
all GPUs on the system support unified shared memory. That's
the case for APUs and MI200 devices when XNACK is enabled.

XNACK can be enabled by setting HSA_XNACK=1 as env var for
supported devices; otherwise, if disable, USM code will
use host fallback.

gcc/ChangeLog:

	* config/gcn/gcn-hsa.h (gcn_local_sym_hash): Fix typo.

include/ChangeLog:

	* hsa.h (HSA_AMD_SYSTEM_INFO_SVM_ACCESSIBLE_BY_DEFAULT): Add
	enum value.

libgomp/ChangeLog:

	* libgomp.texi (gcn): Update USM handling
	* plugin/plugin-gcn.c (GOMP_OFFLOAD_get_num_devices): Handle
	USM if HSA_AMD_SYSTEM_INFO_SVM_ACCESSIBLE_BY_DEFAULT is true.