libc: compile _once in libsys

auxv support requires _once(), but we don't want the libsys version
stomping on the libc version should they diverge in the future. We
could rename it entierly, but for now just hook it in via Makefile.sys.

Reviewed by: kib, emaste, imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/908