Remove contractions and replace 'repo' with 'repository'

@dim can you or somebody else land this? I can't commit it myself.

Also avoid adding _visible.h to SYSINCS

Oh nice! I'll drop this change then.

Here's the branch in sndstat_register returning EINVAL
https://cgit.freebsd.org/src/tree/sys/dev/sound/pcm/sndstat.c#n1170

Hello! I haven't forgotten about this, I've just been fleshing out the code using it. At some point I realized that it's more appropriate to generate the artifacts this change adds (libcore.rlib and libcompiler-builtins.rlib for the kernel arch) as part of each kernel build using the sources in the rust compiler repo. My bindings are pretty much at a state where developers can try it out (and I have a virtio-snd driver demo anyone can already run in QEMU), but it would be nice to have a better solution for getting a rust compiler when building on FreeBSD hosts before announcing it on the mailing lists.

One thing I forgot to note in the commit message is that this option is mutually exclusive with the DOCS options[1]. I'm not sure whether to add a .error if both are set or to just disable the freestanding option and print a .warning or .info message.