AMDGPU: Move isa version and EF_AMDGPU_MACH_* 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).

Differential Revision: https://reviews.llvm.org/D51890

llvm-svn: 341982