[Support/ELF/AMDGPU] Add 32-bit lo/hi got and pc relative relocations

Added relocation names:
  - R_AMDGPU_GOTPCREL32_LO
  - R_AMDGPU_GOTPCREL32_HI
  - R_AMDGPU_REL32_LO
  - R_AMDGPU_REL32_HI

AMDGPU isa only supports 32-bit immediates. In order to access 64-bit address we need to generate 32-bit lo/hi relocations, and do the right math (separate patch). Currently we only generate one 32 bit relocation for lower bits for each access, losing higher bits. Hence we need relocations listed above.

Differential Revision: https://reviews.llvm.org/D25546

llvm-svn: 284191