Syscall support is generated from
//zircon/syscalls. The FIDL files in that directory are first run through
fidlc which produces an intermediate format. That intermediate format is consumed by kazoo which produces output for both the kernel and userspace in a variety of languages. This output includes C or C++ headers for both the kernel and userspace, syscall entry points, other language bindings, and so on.
This tool is invoked as a part of the build, rather than checking in its output.