[OpenMP][AMDGPU] Use DS_Max_Warp_Number instead of WARPSIZE

The size of worker_rootS should have been DS_Max_Warp_Number.
This reduces memory usage by deviceRTL on AMDGPU from around 2.3GB
to around 770MB.

Reviewed By: JonChesterfield, jdoerfert

Differential Revision: https://reviews.llvm.org/D87084