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

Summary: [libomptarget] Move resource id functions into target specific code, implement for amdgcn

Reviewers: jdoerfert, ABataev, grokos

Reviewed By: jdoerfert

Subscribers: jvesely, mgorny, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D71382