LibPthread: Implement pthread_spinlock_t API.

This change implements the pthread user space spinlock API. The
stress-ng Port requires a functioning version to work correctly.

To facilitate the requirements of the posix specification for the API
we implement the spinlock so that the owning tid is the value stored
in the spinlock. This gives us the proper ownership semantics needed
to implement the proper error handling.

Userland/Libraries/LibPthread/pthread.cpp

@@ -559,29 +559,62 @@ int pthread_setcanceltype([[maybe_unused]] int type, [[maybe_unused]] int* oldty
     TODO();
 }
 
-int pthread_spin_destroy([[maybe_unused]] pthread_spinlock_t* lock)
+constexpr static pid_t spinlock_unlock_sentinel = 0;
+int pthread_spin_destroy(pthread_spinlock_t* lock)
 {
-    TODO();
+    auto current = AK::atomic_load(&lock->m_lock);
+
+    if (current != spinlock_unlock_sentinel)
+        return EBUSY;
+
+    return 0;
 }
 
-int pthread_spin_init([[maybe_unused]] pthread_spinlock_t* lock, [[maybe_unused]] int shared)
+int pthread_spin_init(pthread_spinlock_t* lock, [[maybe_unused]] int shared)
 {
-    TODO();
+    lock->m_lock = spinlock_unlock_sentinel;
+    return 0;
 }
 
-int pthread_spin_lock([[maybe_unused]] pthread_spinlock_t* lock)
+int pthread_spin_lock(pthread_spinlock_t* lock)
 {
-    TODO();
+    const auto desired = gettid();
+    while (true) {
+        auto current = AK::atomic_load(&lock->m_lock);
+
+        if (current == desired)
+            return EDEADLK;
+
+        if (AK::atomic_compare_exchange_strong(&lock->m_lock, current, desired, AK::MemoryOrder::memory_order_acquire))
+            break;
+    }
+
+    return 0;
 }
 
-int pthread_spin_trylock([[maybe_unused]] pthread_spinlock_t* lock)
+int pthread_spin_trylock(pthread_spinlock_t* lock)
 {
-    TODO();
+    // We expect the current value to be unlocked, as the specification
+    // states that trylock should lock ony if it is not held by ANY thread.
+    auto current = spinlock_unlock_sentinel;
+    auto desired = gettid();
+
+    if (AK::atomic_compare_exchange_strong(&lock->m_lock, current, desired, AK::MemoryOrder::memory_order_acquire)) {
+        return 0;
+    } else {
+        return EBUSY;
+    }
 }
 
-int pthread_spin_unlock([[maybe_unused]] pthread_spinlock_t* lock)
+int pthread_spin_unlock(pthread_spinlock_t* lock)
 {
-    TODO();
+    auto current = AK::atomic_load(&lock->m_lock);
+
+    if (gettid() != current)
+        return EPERM;
+
+    AK::atomic_store(&lock->m_lock, spinlock_unlock_sentinel);
+    return 0;
 }
 
 int pthread_equal(pthread_t t1, pthread_t t2)