Ignore the PSL_VEC bit in the sigcontext and always copy the bit out from

Ignore the PSL_VEC bit in the sigcontext and always copy the bit out from
the trap frame.  The Altivec unit might have been stolen away from us while
we were running the signal handler, so it is ok that the two don't agree.
Fixes the X server mysteriously exiting (and seemingly freeze).

ok miod@, drahn@