[nfc][libomptarget] Refactor amdgcn target_impl

Summary:
[nfc][libomptarget] Refactor amdgcn target_impl

Removes references to internal libraries from the header
Standardises on C++ mangling for all the target_impl functions
Update comment block
clang-format
Move some functions into a new target_impl.hip source file

This lays the groundwork for implementing the remaining unresolved
symbols in the target_impl.hip source.

Reviewers: jdoerfert, grokos, ABataev, ronlieb

Reviewed By: jdoerfert

Subscribers: jvesely, mgorny, jfb, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D72712