Revert "Revert "[libomptarget] Move resource id functions into target specific...

Revert "Revert "[libomptarget] Move resource id functions into target specific code, implement for amdgcn""

Summary:
This reverts commit dd8a7fcd.

Alexey reports undefined symbols for the new inline functions defined in target_impl.h
This does not reproduce for me for nvptx, or amdgcn, under release or debug builds.

I believe the patch is fine, based on:
 - the semantics of an inline function in C++ (the cuda INLINE functions end
   up as linkonce_odr in IR), which are only legal to drop if they have no uses
 - the code generated from a debug build of clang 9 does not show these undef symbols
 - the tests pass
 - the code is trivial

To progress from here I either need:
 - A tie break - someone to play the role of CI in determining whether the patch works
 - Alexey to provide sufficient information about his build for me to reproduce the failure
 - Alexey to debug why the symbols are disappearing for him and report back

Reviewers: ABataev, jdoerfert, grokos

Subscribers: jvesely, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D71502