Make the `Debug` implementation for `union`s always opaque

[tgross35]

We currently have a lot of unsafe reads of unions fields which is source of easy unsoundness. Let's make things easier while avoiding this completely:

1. Change s_no_extra_traits to do an opaque Debug implementation on unions, here

   libc/src/macros.rs

   Lines 122 to 129 in 2f931d9

   	(it: $(#[$attr:meta])* pub union $i:ident { $($field:tt)* }) => (
   	__item! {
   	#[repr(C)]
   	#[derive(Copy, Clone)]
   	$(#[$attr])*
   	pub union $i { $($field)* }
   	}
   	);

2. Delete all existing Debug implementations on unions (which will now conflict).

The implementation is easy, this will just print the types as some_union { .. }:

#[cfg(feature = "extra_traits")]
impl ::core::fmt::Debug for $i {
    fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
        f.debug_struct(stringify!($i)).finish_non_exhaustive()
    }
}

We can backport this to libc-0.2 since it fixes some soundness issues without breaking any API.