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.

[fuz]

exp-run says it's fine.

[andrew]

lib/libc/aarch64/string/memchr.S

2

Why are you making this weak and not calling memchr directly?

[getz]

lib/libc/aarch64/string/memchr.S

2

I believe we allow users to override functions like memchr but that shouldn't break strlcat. This is also done on the amd64 side.