LibCore: Allow event loops on other threads to wake up

Because the wake pipe is thread-local, it was previously not possible
to wake an event loop across a thread. Therefore, this commit
rearchitects event loop waking by making the wake function a member of
the event loop itself and having it keep a pointer to its thread's wake
pipe. The global wake() function calls wake on the current thread's
event loop.

This also fixes a bug in BackgroundAction: it should wake the event loop
it was created on, instead of the current thread's event loop.

Userland/Libraries/LibThreading/BackgroundAction.h

@@ -63,7 +63,7 @@ private:
                     m_on_complete(m_result.release_value());
                     remove_from_parent();
                 });
-                Core::EventLoop::wake();
+                origin_event_loop->wake();
             } else {
                 this->remove_from_parent();
             }