[lld][AMDGPU] Handle R_AMDGPU_REL16 relocation.

This patch is a followup patch to https://reviews.llvm.org/D105760 which adds this relocation. This handles the relocation in lld.

The s_branch family of instruction does the following:
PC = PC + signext(simm * 4) + 4

so we we do the opposite on the target address before writing it in the instruction stream.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D105761