[libomptarget] Implement wavefront functions for amdgcn

Summary: [libomptarget] Implement wavefront functions for amdgcn

Reviewers: jdoerfert, ABataev, grokos, arsenm

Reviewed By: arsenm

Subscribers: saiislam, wdng, arsenm, jvesely, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D73077