Implement f{16,32,64,128}::{erf,erfc}

Also add
```rust
// #[unstable(feature = "float_gamma", issue = "99842")]
```
to `gamma`-function-related methods on `f16` & `f128`,
as per https://github.com/rust-lang/rust/pull/136324#issuecomment-2626270247

library/std/src/f64.rs

@@ -1151,4 +1151,68 @@ impl f64 {
         let x = unsafe { cmath::lgamma_r(self, &mut signgamp) };
         (x, signgamp)
     }
+
+    /// Error function.
+    ///
+    /// # Unspecified precision
+    ///
+    /// The precision of this function is non-deterministic. This means it varies by platform,
+    /// Rust version, and can even differ within the same execution from one invocation to the next.
+    ///
+    /// This function currently corresponds to the `erf` from libc on Unix
+    /// and Windows. Note that this might change in the future.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(float_erf)]
+    /// /// The error function relates what percent of a normal distribution lies
+    /// /// within `x` standard deviations (scaled by `1/sqrt(2)`).
+    /// fn within_standard_deviations(x: f64) -> f64 {
+    ///     (x * std::f64::consts::FRAC_1_SQRT_2).erf() * 100.0
+    /// }
+    ///
+    /// // 68% of a normal distribution is within one standard deviation
+    /// assert!((within_standard_deviations(1.0) - 68.269).abs() < 0.01);
+    /// // 95% of a normal distribution is within two standard deviations
+    /// assert!((within_standard_deviations(2.0) - 95.450).abs() < 0.01);
+    /// // 99.7% of a normal distribution is within three standard deviations
+    /// assert!((within_standard_deviations(3.0) - 99.730).abs() < 0.01);
+    /// ```
+    #[rustc_allow_incoherent_impl]
+    #[must_use = "method returns a new number and does not mutate the original value"]
+    #[unstable(feature = "float_erf", issue = "136321")]
+    #[inline]
+    pub fn erf(self) -> f64 {
+        unsafe { cmath::erf(self) }
+    }
+
+    /// Complementary error function.
+    ///
+    /// # Unspecified precision
+    ///
+    /// The precision of this function is non-deterministic. This means it varies by platform,
+    /// Rust version, and can even differ within the same execution from one invocation to the next.
+    ///
+    /// This function currently corresponds to the `erfc` from libc on Unix
+    /// and Windows. Note that this might change in the future.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(float_erf)]
+    /// let x: f64 = 0.123;
+    ///
+    /// let one = x.erf() + x.erfc();
+    /// let abs_difference = (one - 1.0).abs();
+    ///
+    /// assert!(abs_difference <= f64::EPSILON);
+    /// ```
+    #[rustc_allow_incoherent_impl]
+    #[must_use = "method returns a new number and does not mutate the original value"]
+    #[unstable(feature = "float_erf", issue = "136321")]
+    #[inline]
+    pub fn erfc(self) -> f64 {
+        unsafe { cmath::erfc(self) }
+    }
 }