Go makes syscalls directly, it doesn't call through the system libc. This does cause support problems on at least NetBSD, don't know what happens on FreeBSD or OpenBSD.
The NetBSD backwards compatibility mechanism is to not change the arguments to an existing syscall, if there is a need to change something then a new syscall will be created and aliased to the old name in the system include files. Old binaries use the old syscall, those compiled after the new one has been added will use that. Languages that don't parse the system .h files like Go need conditional code for NetBSD.
