Use GCC unordered comparison operators

I just noticed that GCC [has unordered comparison operators](https://github.com/rust-lang/gcc/blob/9244ae6e2e96e7d255c73fbd4a9f345e0cd96f35/gcc/tree.def#L861-L870).

It might be more efficient to use those than implementing unordered operators manually [here](https://github.com/rust-lang/rustc_codegen_gcc/blob/fda0bb9588912a3e0606e880ca9f6e913cf8a5a4/src/builder.rs#L1307).