Auto merge of rust-lang#105698 - joboet:unsupported_threads_once, r=thomcc

Use a more efficient `Once` on platforms without threads

The current implementation uses an atomic queue and spins rather than panicking when calling `call_once` recursively. Since concurrency is not supported on platforms like WASM, `Once` can be implemented much more efficiently using just a single non-atomic state variable.

Author: bors