[libomptarget][nfc] Move omp locks under target_impl

Summary:
[libomptarget][nfc] Move omp locks under target_impl

These are likely to be target specific, even down to the lock_t which is
correspondingly moved out of interface.h. The alternative is to include
interface.h in target_impl which substantiatially increases the scope of
those symbols.

The current nvptx implementation deadlocks on amdgcn. The preferred
implementation for that arch is still under discussion - this change
leaves declarations in target_impl.

The functions could be inline for nvptx. I'd prefer to keep the internals
hidden in the target_impl translation unit, but will add the (possibly renamed)
macros to target_impl.h if preferred.

Reviewers: ABataev, jdoerfert, grokos

Reviewed By: jdoerfert

Subscribers: jvesely, mgorny, jfb, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D71574