[Libomptarget][NFC] Move legacy functions to a separate file

This patch moves the old legacy interfaces into `libomptarget` to a
separate file. These do not need to be included anywhere and are simply
provided for backwards compatibility with the ABI. This cleans up the
interface greatly.

Depends on D128817

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D128818