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/Cargo.toml

@@ -36,4 +36,10 @@ check-cfg = [
     # and to stdarch `core_arch` crate which messes-up with Cargo list
     # of declared features, we therefor expect any feature cfg
     'cfg(feature, values(any()))',
+    # Internal features aren't marked known config by default, we use these to
+    # gate tests.
+    'cfg(target_has_reliable_f16)',
+    'cfg(target_has_reliable_f16_math)',
+    'cfg(target_has_reliable_f128)',
+    'cfg(target_has_reliable_f128_math)',
 ]