[OpenMP][AMDGPU] Skip backend and assemble phases for amdgcn
Remove emit-llvm-bc from addClangTargetOptions as it conflicts with -E for save-temps.
AMDGCN does not yet support linking object files so backend and assemble actions are
skipped, leaving LLVM IR as the output format.
Reviewed By: JonChesterfield, ronlieb
Differential Revision: https://reviews.llvm.org/D96769