powerpc: No need to save XER in a system call

The XER is a volatile register so there is no need to save and restore
it over a system call - zero it out in the exception stack frame
instead.

This should fix a 5 cycle stall of the mfxer/std seen on POWER7.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/entry_64.S

@@ -82,16 +82,15 @@ system_call_common:
 	std	r11,GPR10(r1)
 	std	r11,GPR11(r1)
 	std	r11,GPR12(r1)
+	std	r11,_XER(r1)
 	std	r9,GPR13(r1)
 	mfcr	r9
 	mflr	r10
 	li	r11,0xc01
 	std	r9,_CCR(r1)
 	std	r10,_LINK(r1)
 	std	r11,_TRAP(r1)
-	mfxer	r9
 	mfctr	r10
-	std	r9,_XER(r1)
 	std	r10,_CTR(r1)
 	std	r3,ORIG_GPR3(r1)
 	ld	r2,PACATOC(r13)