- don't smash the alt-stack when already using it
- handle jl_critical_error on the original stack, leaving our signal
handling thread free to handle more signals (and helping lock corruption
detection in some cases)
- unblock signals when handling signals: some libc apparently like to
block all signals, which can cause mild havoc, since we'd really like
the user or bad data to be able to still kill the process (and not just
be ignored or cause it to hang)
- reset signals to SIG_DFL earlier (so we recurse less)
- destroy some state from the Task we co-opted to run the exit handlers,
so that it can't accidentally jump back into the running program after
we've started tearing down the process, from an untimely ^C (previously
^C might cancel the exit) or a jlbacktrace call.
- mark functions as leaf with CFI instead of (potentially) smashing the
stack, and add a bit of red-zone if we are recursing (to keep pgcstack
sensible)
- support safe_restore for the mach catch_exception_raise (while we're
trying to generate the backtrace)

(cherry picked from commit 107901d)