[libomptarget] Refactor syncthreads macro to inline function

Summary:
[libomptarget] Refactor syncthreads macro to inline function
See also abandoned D66846, split into this diff and others.

Rev 2 of D66855

Reviewers: jdoerfert, ABataev, grokos, ronlieb, gregrodgers

Subscribers: openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D66861