Support/ELF: Add R_AMDGPU_GOTPCREL relocation

Summary:
We will start generating this in a future patch.

Reviewers: arsenm, kzhuravl, rafael, ruiu, tony-tye

Subscribers: arsenm, llvm-commits, kzhuravl

Differential Revision: http://reviews.llvm.org/D21482

llvm-svn: 273628