[libomptarget][nfc] Extract function from data_sharing, move to common

Summary:
[libomptarget][nfc] Extract function from data_sharing, move to common

Finding the first active thread in the warp is different on nvptx and amdgcn,
mostly due to warp size and the desire for efficiency.

Reviewers: ABataev, jdoerfert, grokos

Reviewed By: jdoerfert

Subscribers: jvesely, mgorny, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D71643