Implement a basic event loop built on LittleLock

It's not guaranteed that there will always be an event loop to run, and this
implementation will serve as an incredibly basic one which does not provide any
I/O, but allows the scheduler to still run.

cc #9128

src/rt/rust_builtin.cpp

@@ -377,6 +377,16 @@ rust_unlock_little_lock(lock_and_signal *lock) {
     lock->unlock();
 }
 
+extern "C" void
+rust_wait_little_lock(lock_and_signal *lock) {
+    lock->wait();
+}
+
+extern "C" void
+rust_signal_little_lock(lock_and_signal *lock) {
+    lock->signal();
+}
+
 typedef void(startfn)(void*, void*);
 
 class raw_thread: public rust_thread {