[ARM] libc: use basic ASM strncat implementation

Summary:
This implementation is the naive strncat implementation
for ARM from netbsd upstream.

Test Plan: Someone else test please, bench gain is significant with http://repo.or.cz/w/glibc.git/blob_plain/ec901dcf2886cfb81d88f9a63cfcd62b9efe6381:/benchtests/bench-strncat.c (no currently integrated bench, only test which is good with this change).

Reviewers: randall.rushing, ConnorBaker, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D492