Add runtime ISA check for amdgcn offloading

Add runtime ISA check for amdgcn offloading

The HSA/ROCm runtime rejects binaries not built for the exact GPU device
present. So far, the libgomp amdgcn plugin does not verify that the GPU ISA
and the ISA specified at compile time match before handing over the binary to
the runtime.  In case of a mismatch, the user is confronted with an unhelpful
runtime error.

This commit implements a runtime ISA check. In case of an ISA mismatch, the
execution is aborted with a clear error message and a hint at the correct
compilation parameters for the GPU on which the execution has been attempted.

libgomp/
	* plugin/plugin-gcn.c (EF_AMDGPU_MACH): New enum.
	* (EF_AMDGPU_MACH_MASK): New constant.
	* (gcn_isa): New typedef.
	* (gcn_gfx801_s): New constant.
	* (gcn_gfx803_s): New constant.
	* (gcn_gfx900_s): New constant.
	* (gcn_gfx906_s): New constant.
	* (gcn_isa_name_len): New constant.
	* (elf_gcn_isa_field): New function.
	* (isa_hsa_name): New function.
	* (isa_gcc_name): New function.
	* (isa_code): New function.
	* (struct agent_info): Add field "device_isa" and remove field
	"gfx900_p".
	* (GOMP_OFFLOAD_init_device): Adapt agent init to "agent_info"
	field changes, fail if device has unknown ISA.
	* (parse_target_attributes): Replace "gfx900_p" by "device_isa".
	* (isa_matches_agent): New function ...
	* (create_and_finalize_hsa_program): ... used from here to check
	that the GPU ISA and the code-object ISA match.