Add new optimized strlen for arm.

Add new optimized strlen for arm.

This optimized version is primarily targeted at cortex-a15.

Tested on all nexus devices using the system/extras/libc_test strlen test.
Tested alignments from 1 to 32 that are powers of 2.
Tested that strlen does not cross page boundaries at all alignments.

Speed improvements listed below:

cortex-a15
- Sizes >= 32 bytes, ~75% improvement.
- Sizes >= 1024 bytes, ~250% improvement.

cortex-a9
- Sizes >= 32 bytes, ~75% improvement.
- Sizes >= 1024 bytes, ~85% improvement.

krait
- Sizes >= 32 bytes, ~95% improvement.
- Sizes >= 1024 bytes, ~160% improvement.

Change-Id: I361b1a36ed89ab991f2a8f0abbf0d7416d39c8f5