libc: link libsys as a auxiliary filter library

At runtime, when rtld loads libc it will also load libsys. For each
symbol that is present in both, the libsys one will override the libc
one. It continues to be the case that program need only link against
libc (usually implicitly). The linkage to libsys is automatic.

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