libc: wrap padding fields with Padding newtype.

[mbyx]

Description

Changes the types of fields with name __pad, _pad, __padding, _padding, __rsvd, __unused to use a new type wrapper around MaybeUninit as per #1453

Only modifies private fields so that the changes can be backported.

Sources

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

[tgross35]

Suggested change

	/// A transparent wrapper over `MaybeUninit<T>` to represent uninitialized padding.
	/// A transparent wrapper over `MaybeUninit<T>` to represent uninitialized padding
	/// while providing `Default`.

To clarify why we don't just use Default

[tgross35]

@rustbot blocked
on ctest2

[tgross35]

@mbyx could you put up a separate PR that only adds the Padding type? I'd like to start directing new PRs to use this.