[ARM] libc: use optimized ASM strncmp implementation

Summary:
This strncmp implementation is based on the current
arm netbsd implementation

Test Plan: https://sourceware.org/git/?p=glibc.git;a=blob;f=benchtests/bench-strncmp.c;h=25df3dbc2eb25346d9a897f8d7a2ada9725584e3;hb=master and integrated bionic-benchmark

Reviewers: randall.rushing

Differential Revision: http://review.codefirex.com/D433