AMDGPU/SI: Add support for AMD code object version 2.

Summary:
Version 2 is now the default.  If you want to emit version 1, use
the amdgcn--amdhsa-amdcov1 triple.

Reviewers: arsenm, kzhuravl

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D19283

llvm-svn: 268647