Unnecessary usage of `intrinsics::copysignf32/64` in `signum`

[Centril]

In f32::signum and f64::signum we use intrinsics::copysignf32 and intrinsics::copysignf64. However, these intrinsics are exposed with a stable wrapper f32::copysign and f64::copysign. We should use those instead.

rust/src/libstd/f32.rs

Line 191 in d132f54

unsafe { intrinsics::copysignf32(1.0, self) }

rust/src/libstd/f64.rs

Line 169 in d132f54

unsafe { intrinsics::copysignf64(1.0, self) }