Auto merge of rust-lang#138087 - tgross35:core-float-math, r=<try>

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], with the exception of `powi` that has been there longer.

Details for each of the changes is in the commit messages.

Tracking issue: rust-lang#137578

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

try-job: aarch64-apple
try-job: aarch64-gnu
try-job: arm-android
tru-job: armhf-gnu
try-job: dist-various-1
try-job: dist-various-2
try-job: i686-msvc-1
try-job: test-various
try-job: x86_64-apple-1
try-job: x86_64-msvc-ext2

Author: bors

compiler/rustc_codegen_cranelift/patches/0027-sysroot_tests-128bit-atomic-operations.patch

@@ -16,10 +16,10 @@ index 1e336bf..35e6f54 100644
 +++ b/coretests/tests/lib.rs
 @@ -2,5 +2,4 @@
  // tidy-alphabetical-start
+ #![cfg_attr(not(bootstrap), feature(cfg_target_has_reliable_f16_f128))]
 -#![cfg_attr(target_has_atomic = "128", feature(integer_atomics))]
  #![cfg_attr(test, feature(cfg_match))]
  #![feature(alloc_layout_extra)]
- #![feature(array_chunks)]
 diff --git a/coretests/tests/atomic.rs b/coretests/tests/atomic.rs
 index b735957..ea728b6 100644
 --- a/coretests/tests/atomic.rs

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)',
 ]