don't declare _start_rust as a "naked" function

The #[naked] attribute means Rust won't emit instructions to push and
pop a new stack frame. But the assembly routine which calls _start_rust
also doesn't push a new stack frame, so there is no valid stack for this
function.

We probably need a stack frame though, since it's implemented in Rust
and we can't easily ensure that Rust won't try to allocate any local
variables. See #5.