adding a syscall to libc?

Hi,

I've started working of a copy_file_range() syscall for FreeBSD. I think I have the
kernel patched and ready for some testing.
However, I'm confused about what I need to do in src/lib/libc/sys?
- Some syscalls have little .c files, but other ones do not.
  When is one of these little .c files needed and, when not needed, what else
  needs to be done? (I notice that syscall.mk in src/sys/sys automagically, but
  I can't see what else, if anything, needs to be done?)

Thanks in advance for your help, rick
ps: I am using the Linux man pages for the syscall ABI. At some point, I'll put this
      in phabricator and post here for comments/review.