Fix 1.88 relnotes

[jieyouxu]

• Use stable std links for impl Default for {*const,*mut} T
• Add missing relnotes for Stabilize slice_as_chunks library feature #139656

  impl [T] {
      const fn as_chunks<const N: usize>(&self) -> (&[[T; N]], &[T]);
      const fn as_rchunks<const N: usize>(&self) -> (&[T], &[[T; N]]);
      const unsafe fn as_chunks_unchecked<const N: usize>(&self) -> &[[T; N]];
      const fn as_chunks_mut<const N: usize>(&mut self) -> (&mut [[T; N]], &mut [T]);
      const fn as_rchunks_mut<const N: usize>(&mut self) -> (&mut [T], &mut [[T; N]]);
      const unsafe fn as_chunks_unchecked_mut<const N: usize>(&mut self) -> &mut [[T; N]];
  }

• Add missing mod ffi::c_str and hint::select_unpredictable

r? @cuviper

[jieyouxu]

Addressed review comments.
@rustbot ready

[cuviper]

Thanks!

@bors r+ rollup

[bors]

📌 Commit 0c1c648 has been approved by cuviper

It is now in the queue for this repository.