[AMDGPU] Provide machine -> name mapping

Summary:
AMDGPU stores a numerical code for the particular GPU variant in EFlags
in the ELF file. This commit provides a mapping from that number into
the machine name for use by objdump-type tools.

Change-Id: Id37fc0bebad443bd89c0080985ce298c4e7e9319

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D46587

llvm-svn: 331798