Rollup merge of #153485 - RalfJung:float-macros-const, r=tgross35

libcore float tests: replace macro shadowing by const-compatible macro

This lets us avoid rust-lang/rust#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

Author: JonathanBrouwer