bfd: add AMDGCN architecture

bfd: add AMDGCN architecture

Add support for the AMDGCN architecture to BFD.

This is the bare minimum to get

  $ ./configure --target=amdgcn-hsa-amdhsa --disable-gas
  $ make all-binutils

working later in this series.

The specific AMDGCN models added here are a bit arbitrary, based on
what we intend to initially support in GDB.  This list will need to be
updated in the future anyway.  The complete up-to-date list of existing
AMDGPU models can be found here:

  https://llvm.org/docs/AMDGPUUsage.html#processors

The ELF format for this architecture is documented here:

  https://llvm.org/docs/AMDGPUUsage.html#elf-code-object

The flags for the "HSA" OS ABI are properly versioned and documented on
that page.  But the NONE, PAL and MESA3D OS ABIs are not well documented
nor versioned.  Taking a peek at the LLVM source code, we see that they
encode their flags the same way as HSA v3.  For example, for PAL:

  https://github.com/llvm/llvm-project/blob/c8b614cd74a92d85936aed5ac7c642af75ffdc29/llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUTargetStreamer.cpp#L601

So at least, we know that all AMDGPU objects (of which AMDGCN objects
are a subset of) at the time of writing encode the specific GPU model in
the EF_AMDGPU_MACH field of e_flags.

bfd/ChangeLog:

	* Makefile.am (ALL_MACHINES, ALL_MACHINES_CFILES):
	Add cpu-amdgcn.c.
	(BFD64_BACKENDS): Add elf64-amdgcn.lo.
	(BFD64_BACKENDS_CFILES): Add elf64-amdgcn.c.
	* Makefile.in: Re-generate.
	* cpu-amdgcn.c: New.
	* elf64-amdgcn.c: New.
	* archures.c (bfd_architecture): Add bfd_arch_amdgcn and related
	mach defines.
	(bfd_amdgcn_arch): New.
	(bfd_archures_list): Add bfd_amdgcn_arch.
	* bfd-in2.h: Re-generate.
	* config.bfd: Handle amdgcn* target.
	* configure.ac: Handle amdgcn_elf64_le_vec.
	* configure: Re-generate.
	* elf-bfd.h (elf_target_id): Add AMDGCN_ELF_DATA.
	* targets.c (amdgcn_elf64_le_vec): New.
	(_bfd_target_vector): Add amdgcn_elf64_le_vec.

include/ChangeLog:

	* elf/amdgpu.h: New.
	* elf/common.h (ELFOSABI_AMDGPU_HSA): Add.

Change-Id: I969f7b14960797e88891c308749a6e341eece5b2