AMDGPU: Re-apply r341982 after fixing the layering issue

Move isa version determination into TargetParser.

Also switch away from target features to CPU string when
determining isa version. This fixes an issue when we
output wrong isa version in the object code when features
of a particular CPU are altered (i.e. gfx902 w/o xnack
used to result in gfx900).

llvm-svn: 342069