libcore no longer builds for AVR

[RalfJung]

When building libcore for a custom target with this target spec, I am getting a build error:

  warning: unused import: `insertion_sort_shift_left`
   --> /Users/runner/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/slice/sort/stable/mod.rs:9:62
    |
  9 |     SMALL_SORT_GENERAL_SCRATCH_LEN, StableSmallSortTypeImpl, insertion_sort_shift_left,
    |                                                              ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_imports)]` on by default
  
  warning: unused import: `crate::slice::sort::shared::smallsort::insertion_sort_shift_left`
   --> /Users/runner/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/slice/sort/unstable/mod.rs:8:5
    |
  8 | use crate::slice::sort::shared::smallsort::insertion_sort_shift_left;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  
  error[E0433]: failed to resolve: use of undeclared crate or module `tiny`
    --> /Users/runner/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/slice/sort/stable/mod.rs:65:13
     |
  65 |             tiny::mergesort(v, scratch, is_less);
     |             ^^^^ use of undeclared crate or module `tiny`

Last known-to-work commit: 6ce3767
First known-to-not-work commit: 1b5aa96

[RalfJung]

Probably caused by #129587
Cc @Voultapher (but you're not expected to fix this since this is a tier 3 target)