[libomptarget][nfc] Move three files under common, build them for amdgcn

Summary:
[libomptarget][nfc] Move three files under common, build them for amdgcn

Change to reduction.cu to remove two dead includes, otherwise no code change.

Reviewers: jdoerfert, ABataev, grokos

Reviewed By: jdoerfert

Subscribers: jvesely, mgorny, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D71601