[AMDGPU] Do not generate spurious PAL metadata

My previous fix rL356591 "[AMDGPU] Added MsgPack format PAL metadata"
accidentally caused a spurious PAL metadata .note record to be emitted
for any AMDGPU output. That caused failures in the lld test
amdgpu-relocs.s. Fixed.

Differential Revision: https://reviews.llvm.org/D59613

Change-Id: Ie04a2aaae890dcd490f22c89edf9913a77ce070e
llvm-svn: 356621