COMMITED UPSTREAM

From: Samuel Thibault <samuel.thibault@ens-lyon.org>
Subject: [PATCH] Fix SS_ONSTACK support

* sysdeps/mach/hurd/i386/sigreturn.c (__sigreturn2): New function,
unlocks SS and returns to the saved PC.
(__sigreturn): Do not unlock SS, and "return" into __sigreturn2 on the
thread stack instead of the saved PC.