ARM: Add optimized ARMv7 strcmp implementation

Add an optimized implementation of strcmp for ARMv7-A cores. This
implementation is significantly faster than the current generic C
implementation, particularly for strings of 16 bytes and longer.

Tested with the glibc string tests for arm-linux-gnueabihf and
armeb-linux-gnueabihf.

The code was written by ARM, who have agreed to assign the copyright
to the FSF for integration into glibc.

ChangeLog:

2014-05-09  Will Newton  <will.newton@linaro.org>

	* sysdeps/arm/armv7/strcmp.S: New file.
	* NEWS: Mention addition of ARMv7 optimized strcmp.