LibPthread: Reimplement condition variables

This implementation features a fast path for pthread_cond_signal() and
pthread_cond_broadcast() for the case there's no thread waiting, and
does not exhibit the "thundering herd" issue in
pthread_cond_broadcast().

Fixes https://github.com/SerenityOS/serenity/issues/8432

Userland/Libraries/LibPthread/CMakeLists.txt

@@ -1,6 +1,7 @@
 set(SOURCES
     forward.cpp
     pthread.cpp
+    pthread_cond.cpp
     pthread_once.cpp
     semaphore.cpp
 )