Use intrinsics for abs and copysign when available

Currently our implementations for `abs` and `copysign` are defined on
the trait, and these are then called from `generic`. It would be better
to call core's `.abs()` / `.copysign(y)`, but we can't do this in the
generic because calling the standalone function could become recursive
(`fabsf` becomes `intrinsics::fabsf32`, that may lower to a call to
`fabsf`).

Change this so the traits uses the call to `core` if available, falling
back to a call to the standalone generic function.

In practice the recursion isn't likely to be a problem since LLVM
probably always lowers `abs`/`copysign` to assembly, but this pattern
should be more correct for functions that we will add in the future
(e.g. `fma`).

This should eventually be followed by a change to call the trait methods
rather than `fabs`/`copysign` directly.

library/compiler-builtins/libm/crates/libm-test/src/f8_impl.rs

@@ -70,6 +70,14 @@ impl Float for f8 {
         Self(a)
     }
 
+    fn abs(self) -> Self {
+        libm::generic::fabs(self)
+    }
+
+    fn copysign(self, other: Self) -> Self {
+        libm::generic::copysign(self, other)
+    }
+
     fn normalize(_significand: Self::Int) -> (i32, Self::Int) {
         unimplemented!()
     }