lib/libc/amd64/string: implement strlcat() through strlcpy()

This should pick up our optimised memchr(), strlen(), and strlcpy()
when strlcat() is called.

Tested by: developers@, exp-run
Approved by: mjg
MFC after: 1 month
MFC to: stable/14
PR: 275785
Differential Revision: https://reviews.freebsd.org/D42863