fasttrap hooks the user breakpoint handler. When no probes are enabled,
it sets the hook pointers to NULL to avoid unnecessary calls into
fasttrap. However, it was missing synchronization for the case where a
user thread hit a breakpoint, and the probe was disabled and hooks
cleared before the thread managed to load the hook.
Take advantage of the fact that interrupts are disabled upon entry to
the breakpointer handler to fix this: when handling a breakpoint, load
the hook pointer before enabling interrupts, and when disabling probes,
issue an SMP rendezvous after removing breakpoints and before clearing
the hook pointers.