[nfc][omptarget] Use builtin var abstraction. Second pass at D69476

Summary:
[nfc][omptarget] Use builtin var abstraction. Second pass at D69476

Use the wrappers in support.h for cuda builtin variables at all call sites.
Localises use of cuda and removes WARPSIZE==32 assumption in debug.h.

Reviewers: ABataev, jdoerfert, grokos

Reviewed By: jdoerfert

Subscribers: openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D69693