[OpenMP][deviceRTLs] Separate declaration of target dependent functions from `target_impl.h`

This patch created a new header file `target_interface.h` for declarations of all target dependent functions. All future targets can get things work by simply implementing all functions declared in the header and macros/data same as each `target_impl.h`.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D95300