Reduce float C99 CRT requirements

[seritools]

Continuing from #36

rust-lang#137578 lets core provide its own impls for various float math functions, removing the need for C99-level CRT (see this comment) for these.

Might want to just enable this when compiling for rust9x.

[seritools]

Oh, since Rust provides these as weakly-linked symbols already, it might just work™ after rebasing!

[seritools]

Oh, they are actually not available on windows :/