add macros for generating Default impls

[telcharr]

Description

Adds impl_default!, s_with_default!, and s_no_extra_traits_with_default! as a start for #4975. Unions get no implicit Default so a union field would need to supply one via #[custom_default(unsafe { mem::zeroed::<U>() })]. As discussed, custom_default needs to be the first attribute on a field since the macro matches it literally.

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

@rustbot label +stable-nominated