sysent: add a NOLIB modifer to prevent stub generation

The yield system call has long existed, but never had a stub. Replace
the hardcoded checks for it in libsys_h.lua and syscalls_map.lua and
stop inserting it into MIASM (requiring libsys/Makefile.sys to disable
the stub).

(This seems like overkill, but I've got another case in CheriBSD so this
reduces my diff appreciably.)

Reviewed by: emaste
Pull Request: https://github.com/freebsd/freebsd-src/pull/1503