[libomptarget][nfc] Move GetWarp/LaneId functions into per arch code

Summary:
[libomptarget][nfc] Move GetWarp/LaneId functions into per arch code

No code change for nvptx. Amdgcn currently has two implementations of GetLaneId,
this patch keeps the one a colleague considered to be superior for our ISA.

GetWarpId is currently the same function for amdgcn and nvptx, but I think it's
cleaner to keep it grouped with all the others than to keep it in support.cu.

Reviewers: jdoerfert, grokos, ABataev

Reviewed By: jdoerfert

Subscribers: jvesely, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D75587