libcore float tests: replace macro shadowing by const-compatible macro This lets us avoid https://github.com/rust-lang/rust/issues/153478. However this means we generate 3 function items per assertion -- or rather, 3*8, since every assertion gets duplicated 8 times (4 float types, each in a const and a non-const variant). That's a lot; is it enough to be concerned about? coretest already takes forever to build. In a quick test, build time increased from 29.8s to 30.8s, but that may also entirely be noise. r? @tgross35