posix: restore original SEGV signal handler upon first entry of our handler
posix: restore original SEGV signal handler upon first entry of our handler

The current SEGV handling on posix is not useful: as soon as our handler
is left, it's triggered again, infinitely.
This patch changes to restore the original handler, so the OS can create
a core dump, etc.

acfloria/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,PX4/Firmware,krbeverx/Firmware,dagar/Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,dagar/Firmware,dagar/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,dagar/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,PX4/Firmware,acfloria/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,krbeverx/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware