[OpenMP] Add thread limit environment variable support to plugins

The OpenMP 5.1 standard defines the environment variable
`OMP_TEAMS_THREAD_LIMIT` to limit the number of threads that will be run in a
single block. This patch adds support for this into the AMDGPU and CUDA
plugins.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D103923