[GCN] Handle generic ISA names in libgomp's plugin-gcn.c

libgomp/ChangeLog:

	* plugin/plugin-gcn.c (ELFABIVERSION_AMDGPU_HSA_V6,
	EF_AMDGPU_GENERIC_VERSION_V, EF_AMDGPU_GENERIC_VERSION_OFFSET,
	GET_GENERIC_VERSION): New #define.
	(elf_gcn_isa_is_generic): New.
	(isa_matches_agent): Accept all generic code objects on the first
	go; extend the diagnostic and handle runtime-failed case.
	(create_and_finalize_hsa_program): Call it also after loading
	the code failed, pass the status.