When dealing with the number of Altivec registers (VR), we should use
the NVRREG constant defined in system file
/usr/include/powerpc64le-linux-gnu/sys/ucontext.h.

However this constant take in account the extra quad word containing
vrsave in split vectors so we must remove 1 to get the exact number of
registers VR.
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>