[coop] Checked build GC critical regions.

[coop] Checked build GC critical regions.

For local handles in coop suspend configuartions we need the idea of a
"critical region": a situation when transitioning from GC
Unsafe (running) to GC Safe (blocking) mode constitutes a bug. In
checked builds, we'd like to assert when that happens.

This PR adds MONO_PREPARE_GC_CRITICAL_REGION,
MONO_FINISH_GC_CRITICAL_REGION and MONO_REQ_GC_NOT_CRITICAL.

In regular build, these are nops. In checked build, they use a new
thread-local flag to prevent transitions from running to blocking.