Initial implementation of core_float_math

Since [1], `compiler-builtins` makes a certain set of math symbols
weakly available on all platforms. This means we can begin exposing some
of the related functions in `core`, so begin this process here.

It is not possible to provide inherent methods in both `core` and `std`
while giving them different stability gates, so standalone functions are
added instead. This provides a way to experiment with the functionality
while unstable; once it is time to stabilize, they can be converted to
inherent.

For `f16` and `f128`, everything is unstable so we can move the inherent
methods.

The following are included to start:

* floor
* ceil
* round
* round_ties_even
* trunc
* fract
* mul_add
* div_euclid
* rem_euclid
* powi
* sqrt
* abs_sub
* cbrt

These mirror the set of functions that we have in `compiler-builtins`
since [1].

Tracking issue: rust-lang#137578

[1]: rust-lang/compiler-builtins#763

Author: tgross35

library/core/src/num/f128.rs

@@ -0,0 +1,11 @@
+//! Bindings to math functions provided by the system `libm` or by the `libm` crate, exposed
+//! via `compiler-builtins`.
+
+// SAFETY: These symbols have standard interfaces in C and are defined by `libm`, or are
+// provided by `compiler-builtins` on unsupported platforms.
+unsafe extern "C" {
+    pub(crate) safe fn cbrt(n: f64) -> f64;
+    pub(crate) safe fn cbrtf(n: f32) -> f32;
+    pub(crate) safe fn fdim(a: f64, b: f64) -> f64;
+    pub(crate) safe fn fdimf(a: f32, b: f32) -> f32;
+}

library/core/src/num/f16.rs

@@ -46,6 +46,7 @@ mod uint_macros; // import uint_impl!
 mod error;
 mod int_log10;
 mod int_sqrt;
+pub(crate) mod libm;
 mod nonzero;
 mod overflow_panic;
 mod saturating;