[libomptarget][nfc] Add declarations of atomic functions for amdgcn

Summary:
[libomptarget][nfc] Add declarations of atomic functions for amdgcn

This enables building more source for amdgcn. The functions are usually available
in a hip runtime header, but are duplicated here to decouple the implementation

Reviewers: jdoerfert, ABataev, grokos

Reviewed By: jdoerfert

Subscribers: jvesely, mgorny, jfb, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D71412