arm64: klib: bitops: fix unpredictable stxr usage

We're currently relying on unpredictable behaviour in our testops
(test_and_*_bit), as stxr is unpredictable when the status register and
the source register are the same

This patch changes reallocates the status register so as to bring us back into
the realm of predictable behaviour. Boot tested on an AEMv8 model.
Signed-off-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

arch/arm64/lib/bitops.S

@@ -50,8 +50,8 @@ ENTRY(	\name	)
 1:	ldxr	x2, [x1]
 	lsr	x0, x2, x3		// Save old value of bit
 	\instr	x2, x2, x4		// toggle bit
-	stxr	w2, x2, [x1]
-	cbnz	w2, 1b
+	stxr	w5, x2, [x1]
+	cbnz	w5, 1b
 	smp_dmb	ish
 	and	x0, x0, #1
 3:	ret