[POWERPC] spufs: Fix missing stop-and-signal

When there is pending signals, current spufs_run_spu() always returns
-ERESTARTSYS and it is called again automatically.
But, if spe already stopped by stop-and-signal or halt instruction,
returning -ERESTARTSYS makes stop-and-signal/halt lost and
spu run over the end-point.

For your convenience, I attached a sample code to restage this bug.
If there is no bug, printed NPC will be 0x4000.

Signed-off-by: Masato Noguchi <Masato.Noguchi@jp.sony.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>