lib/libc/amd64/string: implement strncpy() by calling stpncpy()

Sponsored by: The FreeBSD Foundation
Tested by: developers@, exp-run
Approved by: mjg
MFC after: 1 month
MFC to: stable/14
PR: 275785
Differential Revision: https://reviews.freebsd.org/D42519

(cherry picked from commit e19d46c808267f53455e96a28ff7654211523d2c)