[libomptarget][nvptx][nfc] Move target_impl functions out of header

[libomptarget][nvptx][nfc] Move target_impl functions out of header

This removes most of the differences between the two target_impl.h.

Also change name mangling from C to C++ for __kmpc_impl_*_lock.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D94728