Use const-initialized thread locals (#3923)

This was a relatively recent feature added to the Rust standard library
which should help accelerate calls into WebAssembly slightly.

cranelift/codegen/src/timing.rs

@@ -176,7 +176,7 @@ mod details {
 
     // Information about passes in a single thread.
     thread_local! {
-        static CURRENT_PASS: Cell<Pass> = Cell::new(Pass::None);
+        static CURRENT_PASS: Cell<Pass> = const { Cell::new(Pass::None) };
         static PASS_TIME: RefCell<PassTimes> = RefCell::new(Default::default());
     }