[Synchronization] Skip atomic operations in single-threaded mode on WebAssembly

Use of atomics instructions requires the support of threads proposal and
it's not widely supported yet. So we should enable actual atomic
operations only when targeting wasm32-uknown-wasip1-threads.

Author: kateinoigakukun

stdlib/public/Synchronization/Mutex/WasmImpl.swift

@@ -25,18 +25,22 @@ internal func _swift_stdlib_wake(on: UnsafePointer<UInt32>, count: UInt32) -> UI
 
 extension Atomic where Value == _MutexHandle.State {
   internal borrowing func _wait(expected: _MutexHandle.State) {
+    #if _runtime(_multithreaded)
     _ = _swift_stdlib_wait(
       on: .init(_rawAddress),
       expected: expected.rawValue,
 
       // A timeout of < 0 means indefinitely.
       timeout: -1
     )
+    #endif
   }
 
   internal borrowing func _wake() {
+    #if _runtime(_multithreaded)
     // Only wake up 1 thread
     _ = _swift_stdlib_wake(on: .init(_rawAddress), count: 1)
+    #endif
   }
 }