LibPthread: Implement a basic first pthread mutex

This patch adds these API's:

- pthread_mutex_init()
- pthread_mutex_lock()
- pthread_mutex_unlock()

No mutex attributes are supported yet, so we only do the simplest mutex
wihout recursive locking.

Libraries/LibPthread/pthread.cpp

@@ -1,7 +1,9 @@
+#include <AK/Atomic.h>
 #include <AK/StdLibExtras.h>
 #include <Kernel/Syscall.h>
 #include <pthread.h>
 #include <unistd.h>
+#include <stdio.h>
 
 extern "C" {
 
@@ -27,4 +29,31 @@ int pthread_join(pthread_t thread, void** exit_value_ptr)
     int rc = syscall(SC_join_thread, thread, exit_value_ptr);
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }
+
+int pthread_mutex_init(pthread_mutex_t* mutex, const pthread_mutexattr_t* attributes)
+{
+    // FIXME: Implement mutex attributes
+    UNUSED_PARAM(attributes);
+    *mutex = 0;
+    return 0;
+}
+
+int pthread_mutex_lock(pthread_mutex_t* mutex)
+{
+    auto* atomic = reinterpret_cast<Atomic<u32>*>(mutex);
+    for (;;) {
+        u32 expected = false;
+        if (atomic->compare_exchange_strong(expected, true, AK::memory_order_acq_rel))
+            return 0;
+        sched_yield();
+    }
+}
+
+int pthread_mutex_unlock(pthread_mutex_t* mutex)
+{
+    auto* atomic = reinterpret_cast<Atomic<u32>*>(mutex);
+    atomic->store(false, AK::memory_order_release);
+    return 0;
+}
+
 }