[libomptarget][nfc] Move timer functions behind target_impl

Summary: [libomptarget][nfc] Move timer functions behind target_impl

Reviewers: jdoerfert, ABataev, grokos

Reviewed By: jdoerfert

Subscribers: jvesely, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D71584