[libomptarget][nfc] Use cuda variable wrappers from support.h

Summary:
[libomptarget][nfc] Use cuda variable wrappers from support.h
Reimplementation of D69693, after the revert of D69885

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/D70186