sys: use Option's unwrap_unchecked()

Now that rust-lang#81383 is available,
start using it.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

library/std/src/lib.rs

@@ -290,6 +290,7 @@
 #![feature(nonnull_slice_from_raw_parts)]
 #![feature(once_cell)]
 #![feature(auto_traits)]
+#![feature(option_result_unwrap_unchecked)]
 #![feature(or_patterns)]
 #![feature(panic_info_message)]
 #![feature(panic_internals)]

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

@@ -86,7 +86,7 @@ pub fn my_id() -> u32 {
         if MY_ID == 0 {
             let mut cur = NEXT_ID.load(SeqCst);
             MY_ID = loop {
-                let next = cur.checked_add(1).unwrap_or_else(|| crate::arch::wasm32::unreachable());
+                let next = cur.checked_add(1).unwrap_unchecked();
                 match NEXT_ID.compare_exchange(cur, next, SeqCst, SeqCst) {
                     Ok(_) => break next,
                     Err(i) => cur = i,