Provide string functions for use before ifuncs get resolved.

The change is a no-op for architectures which don't ifunc memset,
memcpy nor memmove.

Convert places which need them. Xen bits by royger.

Reviewed by: kib
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17487