I wonder if there could be some way to sign a dynamic library to allow it to create direct system calls and then pass that as a kernel command line argument at boot?
It is not the only operating system in the "unstable kernel interface" group though. Linux is actually the only one with a stable system call interface.
I've written somewhat at length about this:
Forcing usage of libc is actually an effort towards type safety. `syscall` just takes a syscall number and an unspecified amount of arguments of unconstrained type. It's the opposite of being type safe.
Forcing libc has absolutely nothing to do with type safety. How do you think they invoke syscalls in libc? There's no more type checking of syscalls there than there is in Go's standard library. From a user's point of view it's identical, except you don't have to worry about endless glibc version errors with Go.
The libc can enforce some minimal sanity checks on the syscall arguments. For example the number of arguments and some early validation to avoid entering the kernel if failure can be foreseen. But the main utility is to make it more difficult to use Return Oriented Programming to build exploits.