[AMDGPU][MC][CODEOBJECT] Added predefined symbols to access GPU minor and stepping numbers

Added the following Code Object v3 symbols:
    .amdgcn.gfx_generation_minor
    .amdgcn.gfx_generation_stepping

Reviewers: artem.tamazov, kzhuravl

Differential Revision: https://reviews.llvm.org/D57826

llvm-svn: 353515