Add ARMv7 optimized strlen()

Merge the ARM optimized strlen() routine from Linaro.  Although it is
optimized for ARM Cortex-A9, the performance is still reasonably faster
than the original on Cortex-A8 machines.

Reference benchmark on Nexus S (ARM Cortex-A8; 1 GHz):

[before]
             prc thr   usecs/call      samples   errors cnt/samp     size
strlen_1k      1   1      1.31712           97        0     1000     1024

[after]
             prc thr   usecs/call      samples   errors cnt/samp     size
strlen_1k      1   1      1.05855           96        0     1000     1024

Change-Id: I33835b65bfed434279cfcd34efd848de40c69dfe