Mutex::try_lock doesn't error in wasm32-emscripten

Mutex::try_lock doesn't produce an Err in wasm32-emscripten.

use std::sync::Mutex;
fn main() {
    let m: Mutex<u32> = Mutex::new(1);
    let a = m.try_lock().unwrap();
    let b = m.try_lock().unwrap();
    println!("a + b = {}", *a + *b)
}

In x64, compiling and running this produces:

thread 'main' panicked at try_lock.rs:6:26:
called `Result::unwrap()` on an `Err` value: "WouldBlock"

When built with --target=wasm32-unknown-emscripten (default non-pthreads) there is no error:

a + b = 2

Discovered by @joelburton in https://github.com/curiousdannii/remglk-rs/pull/11

Start by reproducing the example with the wasm32-unknown-emscripten target in its default non-pthreads configuration, then compare Mutex::try_lock behavior with the x64 result described here. The issue names no source file or test; done means the second try_lock produces the expected Err rather than allowing both locks and printing a + b = 2.