lib/libc/aarch64/string: add strlcat SIMD implementation

[getz]

Summary

This patch requires D46243 as it depends on strlcpy being labeled
__strlcpy.

It's a direct copy from the amd64 string functions using memchr and
strlcpy to implement strlcat.

Test Plan

Passes all tests

[getz]

remove manpage update, lets handle that in a separate commit

[andrew]

Could this be made machine independent?

[fuz]

@andrew If we don't have assembly implementations for strlcpy and memchr, it's generally faster to implement this function directly.