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

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>