[docs] Update AMDGPU relocation information

  - Added new notation for specifying relocation calculation
  - Renamed:
    - R_AMDGPU_32_LOW -> R_AMDGPU_ABS32_LO
    - R_AMDGPU_32_HIGH -> R_AMDGPU_ABS32_HI
    - R_AMDGPU_64 -> R_AMDGPU_ABS64
  - Added:
    - R_AMDGPU_REL32
    - R_AMDGPU_REL64
    - R_AMDGPU_ABS32
  - Updated calculations for relative relocations

Differential Revision: http://reviews.llvm.org/D21215

llvm-svn: 272684