This version of signal (which does not rely on a notion of userland
processes and is thus excluded from Vhex) follows C99 semantics but does
not generate any signals by default.
Basically, the signal function sets up function pointers and the signal
function calls them. Termination signals call exit() while other signals
call _Exit(), which is a quicker program termination similar to abort().
C99 allows programs to long jump out of signal handlers (!) which is
unbelievably scary because it would bypass stack switching code in Vhex
as well as normal interrupt handler termination in gint.
This is implemented for gint only currently; on Vhex, _Exit() is likely
just going to be a syscall. For CASIOWIN, this is slightly more
difficult, as there is no native exit syscall.
