binutils/readelf: handle AMDGPU OS ABIs

When the machine is EM_AMDGPU, handle the various OS ABIs described
here:

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

For a binary with the HSA OS ABI, the change looks like:

-  OS/ABI:                            <unknown: 40>
+  OS/ABI:                            AMD HSA

binutils/ChangeLog:

	* readelf.c (get_osabi_name): Handle EM_AMDGPU OS ABIs.

include/ChangeLog:

	* elf/common.h (ELFOSABI_AMDGPU_PAL, ELFOSABI_AMDGPU_MESA3D):
	New.

Change-Id: I383590c390f7dc2fe0f902f50038735626d71863