Inline get_sp_limit/set_sp_limit/get_sp

As per #2521. Inlining seems to improve performance slightly:

                 Inlined          Not Inlined
    x86:         13.5482            14.4112
    x86_64:      17.4712            18.0696

(Average of 5 runs timed with `time`)

```Rust

fn foo() -> int {
    int::from_str(~"28098").unwrap()
}

fn main() {
    for 1000000.times {
        foo();
        foo();
        foo();
        foo();
        foo();
    }
}
```

All run on:

    Linux 3.2.0-0.bpo.4-amd64 #1 SMP Debian 3.2.35-2~bpo60+1 x86_64 GNU/Linux

The MIPS and ARM bits I didn't inline since I'm not as familiar with them and I also can't test them. All green on try.