[runtime] Make suspend work during thread cleanup. Fixes #14555

SDB does suspend/resume during thread cleanup. It used to work until I introduced
critical regions. This won't work anymore as we need to lookup the thread info object
and during cleanup it will be gone.

The fix is to introduce mono_thread_info_current_slow which is a variant of
mono_thread_info_current that does search the thread list if the TLS key
is gone.