Provide ABI modules hooks for process exec/exit and thread exit.

Exec and exit are same as corresponding eventhandler hooks.

Thread exit hook is called somewhat earlier, while thread is still
owned by the process and enough context is available. Note that the
process lock is owned when the hook is called.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D27309