[AMDGPU] do not generate .AMDGPU.config for amdpal os type

Summary:
Now we generate PAL metadata for the amdpal os type, there is no need to
generate the .AMDGPU.config section.

Reviewers: arsenm, nhaehnle, dstuttard

Subscribers: kzhuravl, wdng, yaxunl, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D37760

Change-Id: I303c5fad66656ce97293da60621afac6595b4c18
llvm-svn: 324346