amdgcn, libgomp: USM allocation update

amdgcn, libgomp: USM allocation update

Allocate Unified Shared Memory via malloc and hsa_amd_svm_attributes_set,
instead of hsa_allocate_memory.  This scheme should be more efficient for
for memory that is first accessed by the CPU.

libgomp/ChangeLog:

	* plugin/plugin-gcn.c (HSA_AMD_SYSTEM_INFO_SVM_SUPPORTED): New.
	(HSA_SYSTEM_INFO_SVM_ACCESSIBLE_BY_DEFAULT): New.
	(HSA_AMD_SVM_ATTRIB_GLOBAL_FLAG): New.
	(HSA_AMD_SVM_GLOBAL_FLAG_COARSE_GRAINED): New.
	(hsa_amd_svm_attribute_pair_t): New.
	(struct hsa_runtime_fn_info): Add hsa_amd_svm_attributes_set_fn.
	(dump_hsa_system_info): Dump HSA_AMD_SYSTEM_INFO_SVM_SUPPORTED and
	HSA_SYSTEM_INFO_SVM_ACCESSIBLE_BY_DEFAULT.
	(DLSYM_OPT_FN): New.
	(init_hsa_runtime_functions): Add hsa_amd_svm_attributes_set.
	(GOMP_OFFLOAD_usm_alloc): Use malloc and hsa_amd_svm_attributes_set.
	(GOMP_OFFLOAD_usm_free): Use regular free.
	* testsuite/libgomp.c/usm-1.c: Add -mxnack=on for amdgcn.
	* testsuite/libgomp.c/usm-2.c: Likewise.
	* testsuite/libgomp.c/usm-3.c: Likewise.
	* testsuite/libgomp.c/usm-4.c: Likewise.