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 2fc0717977)

Change-Id: I1ceceb4e745fd68e9d946f96d1d42e0cdaff6ccf

libc/arch-arm/arm.mk

@@ -15,7 +15,6 @@ _LIBC_ARCH_COMMON_SRC_FILES := \
     arch-arm/bionic/setjmp.S \
     arch-arm/bionic/sigsetjmp.S \
     arch-arm/bionic/strcpy.S \
-    arch-arm/bionic/strlen.c.arm \
     arch-arm/bionic/syscall.S \
     arch-arm/bionic/tgkill.S \
     arch-arm/bionic/tkill.S \