Opt-in busywait mode for futexes

[kateinoigakukun]

We are heavily using wasi-libc + wasip1-threads on browsers. Since wasi-libc uses memory.atomic.wait32 to implement futex, and the usage of memory.atomic.wait32 on the main thread is prohibited on browsers, we currently need to write code carefully not to reach the instruction.

Emscripten addresses this limitation by employing a busy-wait on the main thread as a workaround. Rust has always employs busywait regardless of the current thread is main. This approach, while effective for browsers, introduces unnecessary overhead in environments where memory.atomic.wait32 is permitted.

This change provides a similar solution by introducing an opt-in busy-wait mode for futexes. By making this an optional feature, we avoid imposing the overhead of busy-waiting in non-browser environments while ensuring compatibility with browser-based restrictions.

#include <wasi/libc-busywait.h>

/// Enable busywait in futex on current thread.
void __wasilibc_enable_futex_busywait_on_current_thread(void);

This change slightly adds some runtime overheads in futex to check if we should use busywait, but it can be optimized away as long as __wasilibc_enable_futex_busywait_on_current_thread is not used by user program and LTO is enabled.