Support #![feature(unchecked_math)]

The following code:
```
#![feature(unchecked_math)]

pub fn nop(num: i32) -> i32 {
    unsafe { (num.unchecked_mul(2)) / 2 }
}
```
gives:
```
example::nop:
        lea     eax, [rdi+rdi]
        sar     eax
        ret
```
while cg_llvm gives:
```
example::nop:
        mov     eax, edi
        ret
```
because it seems cg_gcc always enable checked maths.