Add more f16 and f128 library functions and constants

This adds everything that was directly or transitively blocked on const
arithmetic for these types, which was recently merged.

Since const arithmetic is recent, most of these need to be gated by
`bootstrap`.

Anything that relies on intrinsics that are still missing is excluded.

library/std/src/f16.rs

@@ -32,4 +32,33 @@ impl f16 {
     pub fn powi(self, n: i32) -> f16 {
         unsafe { intrinsics::powif16(self, n) }
     }
+
+    /// Computes the absolute value of `self`.
+    ///
+    /// This function always returns the precise result.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(f16)]
+    /// # #[cfg(reliable_f16)] {
+    ///
+    /// let x = 3.5_f16;
+    /// let y = -3.5_f16;
+    ///
+    /// assert_eq!(x.abs(), x);
+    /// assert_eq!(y.abs(), -y);
+    ///
+    /// assert!(f16::NAN.abs().is_nan());
+    /// # }
+    /// ```
+    #[inline]
+    #[cfg(not(bootstrap))]
+    #[rustc_allow_incoherent_impl]
+    #[unstable(feature = "f16", issue = "116909")]
+    #[must_use = "method returns a new number and does not mutate the original value"]
+    pub fn abs(self) -> Self {
+        // FIXME(f16_f128): replace with `intrinsics::fabsf16` when available
+        Self::from_bits(self.to_bits() & !(1 << 15))
+    }
 }