This takes a similar approach to deferred signal handling and allows any given
thread to be interrupted while running JIT code by protecting the appropriate
page as RO. When the thread then enters a new block, it will try to acccess
that page and segfault. This is safer than just sending a signal to the thread
as that could stop in a place where JIT context couldn't be recovered correctly.
It is not an external component, and it makes paths needlessly long.
Ryan seemed amenable to this when we discussed on IRC earlier.
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>