amdgcn: Add gfx908 support

amdgcn: Add gfx908 support

gcc/

	* config/gcn/gcn-opts.h (enum processor_type): Add PROCESSOR_GFX908.
	* config/gcn/gcn.c (gcn_omp_device_kind_arch_isa): Add gfx908.
	(output_file_start): Add gfx908.
	* config/gcn/gcn.opt (gpu_type): Add gfx908.
	* config/gcn/t-gcn-hsa (MULTILIB_OPTIONS): Add march=gfx908.
	(MULTILIB_DIRNAMES): Add gfx908.
	* config/gcn/mkoffload.c (EF_AMDGPU_MACH_AMDGCN_GFX908): New define.
	(main): Recognize gfx908.
	* config/gcn/t-omp-device: Add gfx908.

libgomp/

	* plugin/plugin-gcn.c (EF_AMDGPU_MACH): Add
	EF_AMDGPU_MACH_AMDGCN_GFX908.
	(gcn_gfx908_s): New constant string.
	(isa_hsa_name): Add gfx908.
	(isa_code): Add gfx908.

(cherry picked from commit 3535402e20118655b2ad4085a6e1d4f1b9c46e92)