strlen-armv7.S: Import latest strlen cortex-strings code.

Import the latest version of strlen from the Linaro cortex-strings
package. This version is faster across a variety of block size and
alignments on ARMv7.

newlib/ChangeLog:

2013-06-21  Will Newton  <will.newton@linaro.org>

	* libc/machine/arm/strlen-armv7.S: Import latest strlen
	code from Linaro cortex-strings.