amdgcn: Add gfx90c target

Add support for gfx90c GCN5 APU integrated graphics devices.
The LLVM AMDGPU documentation does not list those devices as supported
by rocm-amdhsa, but it passes most libgomp offloading tests.
Although they are constrainted compared to dGPUs, they might be
interesting for learning, experimentation, and testing.

gcc/ChangeLog:

	* config.gcc: Add gfx90c.
	* config/gcn/gcn-hsa.h (NO_SRAM_ECC): Likewise.
	* config/gcn/gcn-opts.h (enum processor_type): Likewise.
	(TARGET_GFX90c): New macro.
	* config/gcn/gcn.cc (gcn_option_override): Handle gfx90c.
	(gcn_omp_device_kind_arch_isa): Likewise.
	(output_file_start): Likewise.
	* config/gcn/gcn.h: Add gfx90c.
	* config/gcn/gcn.opt: Likewise.
	* config/gcn/mkoffload.cc (EF_AMDGPU_MACH_AMDGCN_GFX90c): New macro.
	(get_arch): Handle gfx90c.
	(main): Handle EF_AMDGPU_MACH_AMDGCN_GFX90c
	* config/gcn/t-omp-device: Add gfx90c.
	* doc/install.texi: Likewise.
	* doc/invoke.texi: Likewise.

libgomp/ChangeLog:

	* plugin/plugin-gcn.c (isa_hsa_name): Handle EF_AMDGPU_MACH_AMDGCN_GFX90c.
	(isa_code): Handle gfx90c.
	(max_isa_vgprs): Handle EF_AMDGPU_MACH_AMDGCN_GFX90c.

Signed-off-by: Frederik Harwath <frederik@harwath.name>