[Clang][Flang][AMDGPU] Add support for AMDGPU to Flang driver

Scope of changes:
  1) Extract common code between Clang and Flang for parsing AMDGPU features
  2) Add function which adds implicit target features for AMDGPU as Clang does
  3) Add AMDGPU target as one of valid targets for Flang

Differential Revision: https://reviews.llvm.org/D145579

Reviewed By: yaxunl, awarzynski