Add OpenACC acc_get_property support for AMD GCN

Add OpenACC acc_get_property support for AMD GCN

Add full support for the OpenACC 2.6 acc_get_property and
acc_get_property_string functions to the libgomp GCN plugin.

libgomp/
	* plugin-gcn.c (struct agent_info): Add fields "name" and
	"vendor_name" ...
	(GOMP_OFFLOAD_init_device): ... and init from here.
	(struct hsa_context_info): Add field "driver_version_s" ...
	(init_hsa_contest): ... and init from here.
	(GOMP_OFFLOAD_openacc_get_property): Replace stub with a proper
	implementation.
	* testsuite/libgomp.oacc-c-c++-common/acc_get_property.c:
	Enable test execution for amdgcn and host offloading targets.
	* testsuite/libgomp.oacc-fortran/acc_get_property.f90: Likewise.
	* testsuite/libgomp.oacc-c-c++-common/acc_get_property-aux.c
	(expect_device_properties): Split function into ...
	(expect_device_string_properties): ... this new function ...
	(expect_device_memory): ... and this new function.
	* testsuite/libgomp.oacc-c-c++-common/acc_get_property-gcn.c:
	Add test.