Rollup merge of #106798 - scottmcm:signum-via-cmp, r=Mark-Simulacrum

Implement `signum` with `Ord`

Rather than needing to do things like #105840 for `signum` too, might as well just implement that method using `Ord`, since it's doing the same "I need `-1`/`0`/`+1`" behaviour that `cmp` is already doing.

This also seems to slightly improve the assembly: <https://rust.godbolt.org/z/5oEEqbxK1>

library/core/src/num/int_macros.rs

@@ -2574,12 +2574,13 @@ pub const fn abs_diff(self, other: Self) -> $UnsignedT {
         #[must_use = "this returns the result of the operation, \
                       without modifying the original"]
         #[inline(always)]
+        #[rustc_allow_const_fn_unstable(const_cmp)]
         pub const fn signum(self) -> Self {
-            match self {
-                n if n > 0 =>  1,
-                0          =>  0,
-                _          => -1,
-            }
+            // Picking the right way to phrase this is complicated
+            // (<https://graphics.stanford.edu/~seander/bithacks.html#CopyIntegerSign>)
+            // so delegate it to `Ord` which is already producing -1/0/+1
+            // exactly like we need and can be the place to deal with the complexity.
+            self.cmp(&0) as _
         }
 
         /// Returns `true` if `self` is positive and `false` if the number is zero or