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.

Merge from internal master.

(cherry-picked from 2fc071797743b88a9a47427d46baed7c7b24f4d2)

Change-Id: I1ceceb4e745fd68e9d946f96d1d42e0cdaff6ccf

Conflicts:
	libc/arch-arm/arm.mk
	libc/arch-arm/cortex-a15/cortex-a15.mk
	libc/arch-arm/cortex-a9/cortex-a9.mk
	libc/arch-arm/generic/generic.mk
	libc/arch-arm/krait/krait.mk
Fix assembler errors in generic arm strlen.c.

Tested using a static version of the strlen libc_test program
on a nexus7 that uses the generic code.

Merge from internal master.

(cherry-picked from d8d10a8994472e40d19301b7087806630877b4d5)

Change-Id: I88f7dc01dc5b5c3ac2d5580d92153bc1bc36c564

Conflicts:
	libc/arch-arm/generic/bionic/strlen.c