[OpenMP] Do not link the bitcode OpenMP runtime when targeting AMDGPU.

The AMDGPU target can only emit LLVM-IR, so we can always rely on LTO to
link the static version of the runtime optimally. Using the static
library only has a few advantages. Namely, it avoids several known bugs
and allows us to optimize out more functions. This is legal since the
changes in D142486 and D142484

Depends on D142486 D142484

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D142491