Fix NaN nondeterminism

[nhynes]

We want to enable floats, but NaNs in wasm are nondeterministic and can cause consensus failure. We need to comprehensively patch all of the cases in which nondeterminism is visible.

According to LLVM, NaNs can be canonicalized by multiplying by 1. In Rust, it's possible to do this while avoiding constant folding using test::black_box(1f64 * arg).

@Yawning seems to be the resident expert on this topic and should likely be the judge of whether the implementation is satisfactory.

[nhynes]

Most interesting changes would have to happen in runner.rs. Perhaps run_store, run_reinterpret, run_trunc, run_extend, run_const.

I maybe wouldn't worry about run_call to externs since we control those functions. We do have an expf extern, but that can only return Inf if not already given a NaN.

[nhynes]

As a side-note, we don't really need nan-preserving-float since we don't actually use x87 (ref).