Provide naive but self-contained implementations of memset(3) and
bzero(3) for rtld.

This again reduces rtld dependency on libc, and in future, avoid ifunc
relocations when the functions are converted to ifuncs in libc.

Reported by: mjg
Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D18400