[PJRT:CPU] Fix thread-pool stack sizes to 2MiB.
The default thread pool size is too small on Mac OS.
An older version of this runtime based on StreamExecutor set a 2MiB stack size as well, but that change was most likely lost during the TFRT rewrite.
Fixes https://github.com/google/jax/issues/20428
PiperOrigin-RevId: 620853544
