Arm32: correct string.h functions for "int" -> "unsigned char" conversion

While Arm64 does so uniformly, for Arm32 only strchr() currently handles
this properly. Add the necessary conversion also to strrchr(), memchr(),
and memset().

As to the placement in memset(): Putting the new insn at the beginning
of the function is apparently deemed more "obvious". It could be placed
later, as the code reachable without ever making it to the "1" label
only ever does byte stores.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

xen/arch/arm/arm32/lib/memchr.S

@@ -14,6 +14,7 @@
 	.text
 	.align	5
 ENTRY(memchr)
+	and	r1, r1, #0xff
 1:	subs	r2, r2, #1
 	bmi	2f
 	ldrb	r3, [r0], #1

xen/arch/arm/arm32/lib/memset.S

@@ -15,6 +15,7 @@
 	.align	5
 
 ENTRY(memset)
+	and	r1, r1, #0xff
 	ands	r3, r0, #3		@ 1 unaligned?
 	mov	ip, r0			@ preserve r0 as return value
 	bne	6f			@ 1

xen/arch/arm/arm32/lib/strrchr.S

@@ -14,6 +14,7 @@
 		.text
 		.align	5
 ENTRY(strrchr)
+		and	r1, r1, #0xff
 		mov	r3, #0
 1:		ldrb	r2, [r0], #1
 		teq	r2, r1