[runtime] Don't start aborting a thread if it's running any .cctor. Fixes #43320.

[runtime] Don't start aborting a thread if it's running any .cctor. Fixes #43320.

A thread abort must not start if the thread is running a .cctor.
OTOH, if the cctor self aborts, it should start.

Threads now track how many cctors they are running in the cctor_exec_depth variable.

The abort machinery (in async_abort_critical) checks that flag and bail from
starting the abort.

The cctor code (in mono_runtime_class_init_full) ensures that cctor_exec_depth is bumped
around execution of the cctor itself. Additionally, error handling was cleaned up as it was pretty
inconsistent.