[libomptarget][nfc] Move cuda threadfence functions behind kmpc_impl

Summary:
[libomptarget][nfc] Move cuda threadfence functions behind kmpc_impl

Part of building code under common/ without requiring a cuda compiler

Reviewers: ABataev, jdoerfert, grokos

Reviewed By: ABataev

Subscribers: jvesely, jfb, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D71102