ARM: 8671/1: V7M: Preserve registers across switch from Thread to Handler mode

According to ARMv7 ARM, when exception is taken content of r0-r3, r12
is unknown (see ExceptionTaken() pseudocode). Even though existent
implementations keep these register unchanged, preserve them to be in
line with architecture.
Reported-by: Dobromir Stefanov <dobromir.stefanov@arm.com>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

arch/arm/mm/proc-v7m.S

@@ -135,9 +135,11 @@ __v7m_setup_cont:
 	dsb
 	mov	r6, lr			@ save LR
 	ldr	sp, =init_thread_union + THREAD_START_SP
+	stmia	sp, {r0-r3, r12}
 	cpsie	i
 	svc	#0
 1:	cpsid	i
+	ldmia	sp, {r0-r3, r12}
 	str	r5, [r12, #11 * 4]	@ restore the original SVC vector entry
 	mov	lr, r6			@ restore LR