libsys: don't try to expose yield

The undocumented yield system call has never been implemented via libc
or libsys (except accidentally for <15 minutes in 1998 between commits
abd529cebab9 and 0db2fac06ab7). Avoid trying to export it now to avoid
failures when linking with --no-undefined-version.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44236