Instead of relying on creating a fake stack frame, and having no signals
delivered, kernel bugs, accidentally gc_collect, or other issues occur
during the delivery and execution of these calls, use the ability we
added recently to emulate a longjmp into a unw_context to eliminate any
time where there would exist any invalid states.

Secondly, when calling jl_exit_thread0_cb, we used to end up completely
smashing the unwind info (with CFI_NOUNWIND), but this makes core files
from SIGQUIT much less helpful, so we now have a `fake_stack_pop`
function with contains the necessary CFI directives such that a minimal
unwind from the debugger will likely still succeed up into the frames
that were removed. We cannot do this perfectly on AArch64 since that
platform's DWARF spec lacks the ability to do so. On other platforms,
this should be possible to implement exactly (subject to libunwind
implementation quality).