docs: Add AMDGPU relocation information

Summary:
This documents the various relocation types that are supported by the
Radeon Open Compute (ROC) runtime (which is essentially the dynamic
linker for AMDGPU).

Only R_AMDGPU_32 is not currently supported by the ROC runtime, but
it will usually be resolved at link time by lld.

Patch by: Konstantin Zhuravlyov

Reviewers: kzhuravl, rafael

Subscribers: rafael, arsenm, llvm-commits, kzhuravl

Differential Revision: http://reviews.llvm.org/D20952

llvm-svn: 272352