[libomptarget] Implement most hip atomic functions in terms of intrinsics

Summary:
[libomptarget] Implement hip atomic functions in terms of intrinsics

All but atomicInc can be implemented using type generic clang intrinsics.
There is not yet a corresponding intrinsic for atomicInc in clang, only one in
LLVM. This patch leaves atomicInc as an unresolved symbol.

Reviewers: jdoerfert, ABataev, hfinkel, grokos, arsenm

Reviewed By: arsenm

Subscribers: sri, saiislam, wdng, jvesely, mgorny, jfb, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D73076