[libomptarget] [amdgpu] Change default number of teams per computation unit

This patch is related to https://reviews.llvm.org/D98832. Based on discussions there, I decided to separate out the teams default as this patch. This change is to increase the number of teams per computation unit so as to provide more wavefronts for hiding latency. This change improves performance for some programs, including 20-50% for some Stream benchmarks.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D99003