arm: Remove __builtin_arm_uqsub8 usage on string-fza.h

The __builtin_arm_uqsub8 is an internal GCC builtin which might change
in future release (the correct way is to include "arm_acle.h" and use
__uqsub8 ()).  Since not all compilers support it, just use the
inline assembler instead.

Checked on armv7a-linux-gnueabihf.
Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>