macros: forward cfg attributes to generated impls

[telcharr]

Description

Follow-up to #5257 for the cfg issue that was spotted, plus the same on Debug for unions.

Attributes are now split into cfg_attrs and other_attrs, both go on the type, only the cfgs repeat on the impl. Needed them as tt rather than meta so cfg is still matchable.

The generated impls also warn when the type or its fields are deprecated. s_no_extra_traits! got an allow(deprecated) for that but the newer macros never picked it up.

The splitting arms are duplicated between union_with_debug! and struct_with_default!. Sharing them needs a callback macro which felt worse to read, but let me know if you'd rather have that.

Checklist

• Relevant tests in libc-test/semver have been updated
• Commit messages permalink to headers for added or changed API
• Placeholder or unstable values like *LAST or *MAX have the standard
  doc comment
• Tested locally (cargo test -p libc-test --target mytarget);
  especially relevant for platforms that may not be checked in CI

@rustbot label +stable-nominated