Rollup merge of #151016 - fix_wasi_threading, r=alexcrichton

fix: WASI threading regression by disabling pthread usage

PR rust-lang/rust#147572 changed WASI to use the Unix threading implementation, but WASI does not support threading. When the Unix code tries to call pthread_create, it fails with EAGAIN, causing libraries like rayon to panic when trying to initialize their global thread pool.

The old wasip1/wasip2 implementations:
- wasip1: Threading conditionally available with atomics (experimental)
- wasip2: Threading unconditionally unsupported

This fix restores that behavior by disabling pthread-based threading for all WASI targets:
1. Guard the pthread-based Thread implementation with #[cfg(not(target_os = "wasi"))]
2. Provide an unsupported stub (Thread(!)) for WASI
3. Return Err(io::Error::UNSUPPORTED_PLATFORM) when Thread::new is called

Fixes the regression where rayon-based code (e.g., lopdf in PDF handling) panicked on WASI after nightly-2025-12-10.

Before fix:
  pthread_create returns EAGAIN (error code 6)
  ThreadPoolBuildError { kind: IOError(Os { code: 6,
  kind: WouldBlock, message: "Resource temporarily unavailable" }) }

After fix:
  Thread::new returns Err(io::Error::UNSUPPORTED_PLATFORM)
  Libraries can gracefully handle the lack of threading support

r? @alexcrichton

library/std/src/sys/thread/unix.rs

@@ -44,6 +44,15 @@ impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub unsafe fn new(stack: usize, init: Box<ThreadInit>) -> io::Result<Thread> {
+        // FIXME: remove this block once wasi-sdk is updated with the fix from
+        // https://github.com/WebAssembly/wasi-libc/pull/716
+        // WASI does not support threading via pthreads. While wasi-libc provides
+        // pthread stubs, pthread_create returns EAGAIN, which causes confusing
+        // errors. We return UNSUPPORTED_PLATFORM directly instead.
+        if cfg!(target_os = "wasi") {
+            return Err(io::Error::UNSUPPORTED_PLATFORM);
+        }
+
         let data = init;
         let mut attr: mem::MaybeUninit<libc::pthread_attr_t> = mem::MaybeUninit::uninit();
         assert_eq!(libc::pthread_attr_init(attr.as_mut_ptr()), 0);