Fix ThreadId implementation on targets without atomicu64

[dtolnay]

The new ThreadId code from #5329 in tokio 1.24.0 assumes that StaticAtomicU64 has load and compare_exchange_weak methods, which was not always the case. This was causing the following build failure:

error[E0599]: no method named `load` found for struct `StaticAtomicU64` in the current scope
  --> tokio-1.24.0/src/runtime/thread_id.rs:12:32
   |
12 |         let mut last = NEXT_ID.load(Relaxed);
   |                                ^^^^ method not found in `StaticAtomicU64`
   |
  ::: tokio-1.24.0/src/loom/std/atomic_u64_static_once_cell.rs:5:1
   |
5  | pub(crate) struct StaticAtomicU64 {
   | --------------------------------- method `load` not found for this

error[E0599]: no method named `compare_exchange_weak` found for struct `StaticAtomicU64` in the current scope
  --> tokio-1.24.0/src/runtime/thread_id.rs:19:27
   |
19 |             match NEXT_ID.compare_exchange_weak(last, id, Relaxed, Relaxed) {
   |                           ^^^^^^^^^^^^^^^^^^^^^ method not found in `StaticAtomicU64`
   |
  ::: tokio-1.24.0/src/loom/std/atomic_u64_static_once_cell.rs:5:1
   |
5  | pub(crate) struct StaticAtomicU64 {
   | --------------------------------- method `compare_exchange_weak` not found for this

[taiki-e]

Thanks! I can reproduce this with the case where no_atomic_u64 & no_const_mutex_new cfgs are enabled and parking_lot feature is disabled.

RUSTFLAGS='--cfg tokio_no_atomic_u64 --cfg tokio_no_const_mutex_new' \
  cargo build --manifest-path tokio/Cargo.toml --features rt

[M1cha]

where did these functions come from on platforms that do support atomic u64? I don't see any Deref implementation or anything like that.

[taiki-e]

@M1cha This issue is unrelated to targets that support AtomicU64. (On such targets, StaticAtomicU64 is an alias of AtomicU64.)