[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