[libomptarget][amdgcn] Build amdgcn devicertl as openmp

[libomptarget][amdgcn] Build amdgcn devicertl as openmp

Change cmake to build as openmp and fix up some minor errors in the code.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D96533

GitOrigin-RevId: 6f04addc8b2eee0d88b97facfa4fb7424b4b74bd
7 files changed