[libomptarget] Move supporti.h to support.cu

Summary:
[libomptarget] Move supporti.h to support.cu
Reimplementation of D69652, without the unity build and refactors.
Will need a clean build of libomptarget as the cmakelists changed.

Reviewers: ABataev, jdoerfert

Reviewed By: jdoerfert

Subscribers: mgorny, jfb, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D70131