[libomptarget] Build most of common/src for amdgcn

Summary:
[libomptarget] Build most of common/src for amdgcn

Excluding parallel.cu, which uses an integer min() from cuda,
Excluding support.cu, which calls malloc that is not yet available for amdgcn

Reviewers: jdoerfert, ABataev, grokos

Reviewed By: jdoerfert

Subscribers: gregrodgers, ronlieb, jvesely, mgorny, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D71446