[nfc][libomptarget] Refactor nvptx/target_impl.cu

Summary:
[nfc][libomptarget] Refactor nxptx/target_impl.cu

Use __kmpc_impl_atomic_add instead of atomicAdd to match the rest of the file.
Alternatively, target_impl.cu could use the cuda functions directly. Using a mixture in this
file was an oversight, happy to resolve in either direction.

Removed some comments that look outdated.

Call __kmpc_impl_unset_lock directly to avoid a redundant diagnostic and remove an implict
dependency on interface.h.

Reviewers: ABataev, grokos, jdoerfert

Reviewed By: jdoerfert

Subscribers: jfb, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D72719