Add some f32 and f64 inherent methods in libcore

… previously in the unstable core::num::Float trait.

Per https://github.com/rust-lang/rust/issues/32110#issuecomment-379503183,
the `abs`, `signum`, and `powi` methods are *not* included for now
since they rely on LLVM intrinsics and we haven’t determined yet whether
those instrinsics lower to calls to libm functions on any platform.

src/liballoc/vec.rs

@@ -74,6 +74,7 @@ use core::iter::{FromIterator, FusedIterator, TrustedLen};
 use core::marker::PhantomData;
 use core::mem;
 #[cfg(not(test))]
+#[cfg(stage0)]
 use core::num::Float;
 use core::ops::Bound::{Excluded, Included, Unbounded};
 use core::ops::{Index, IndexMut, RangeBounds};