LibGUI+Kernel: Add a GLock class (userspace mutex.)

It's basically a userspace port of the kernel's Lock class.
Added gettid() and donate() syscalls to support the timeslice donation
feature we already enjoyed in the kernel.

LibGUI/GLock.h

@@ -0,0 +1,106 @@
+#pragma once
+
+#include <AK/Assertions.h>
+#include <AK/Types.h>
+#include <unistd.h>
+
+#define memory_barrier() asm volatile ("" ::: "memory")
+
+static inline dword CAS(volatile dword* mem, dword newval, dword oldval)
+{
+    dword ret;
+    asm volatile(
+        "cmpxchgl %2, %1"
+        :"=a"(ret), "+m"(*mem)
+        :"r"(newval), "0"(oldval)
+        :"cc", "memory");
+    return ret;
+}
+
+class GLock {
+public:
+    GLock() { }
+    ~GLock() { }
+
+    void lock();
+    void unlock();
+
+private:
+    volatile dword m_lock { 0 };
+    dword m_level { 0 };
+    int m_holder { -1 };
+};
+
+class GLocker {
+public:
+    [[gnu::always_inline]] inline explicit GLocker(GLock& l) : m_lock(l) { lock(); }
+    [[gnu::always_inline]] inline ~GLocker() { unlock(); }
+    [[gnu::always_inline]] inline void unlock() { m_lock.unlock(); }
+    [[gnu::always_inline]] inline void lock() { m_lock.lock(); }
+
+private:
+    GLock& m_lock;
+};
+
+[[gnu::always_inline]] inline void GLock::lock()
+{
+    for (;;) {
+        if (CAS(&m_lock, 1, 0) == 0) {
+            if (m_holder == -1 || m_holder == gettid()) {
+                m_holder = gettid();
+                ++m_level;
+                memory_barrier();
+                m_lock = 0;
+                return;
+            }
+            m_lock = 0;
+        }
+        dbgprintf("donate to %d\n", m_holder);
+        int rc = donate(m_holder);
+        if (rc < 0)
+            dbgprintf("donate: %s\n", strerror(errno));
+    }
+}
+
+inline void GLock::unlock()
+{
+    for (;;) {
+        if (CAS(&m_lock, 1, 0) == 0) {
+            ASSERT(m_holder == gettid());
+            ASSERT(m_level);
+            --m_level;
+            if (m_level) {
+                memory_barrier();
+                m_lock = 0;
+                return;
+            }
+            m_holder = -1;
+            memory_barrier();
+            m_lock = 0;
+            return;
+        }
+        donate(m_holder);
+    }
+}
+
+#define LOCKER(lock) GLocker locker(lock)
+
+template<typename T>
+class GLockable {
+public:
+    GLockable() { }
+    GLockable(T&& resource) : m_resource(move(resource)) { }
+    GLock& lock() { return m_lock; }
+    T& resource() { return m_resource; }
+
+    T lock_and_copy()
+    {
+        LOCKER(m_lock);
+        return m_resource;
+    }
+
+private:
+    T m_resource;
+    GLock m_lock;
+};
+