Fix clippy::thread_local_initializer_can_be_made_const

[andersk]

warning: initializer for `thread_local` value can be made `const`
  --> gc/src/gc.rs:29:52
   |
29 | thread_local!(pub static GC_DROPPING: Cell<bool> = Cell::new(false));
   |                                                    ^^^^^^^^^^^^^^^^ help: replace with: `const { Cell::new(false) }`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#thread_local_initializer_can_be_made_const
   = note: `#[warn(clippy::thread_local_initializer_can_be_made_const)]` on by default