LibPthread: Start working on a POSIX threading library

This patch adds pthread_create() and pthread_exit(), which currently
simply wrap our existing create_thread() and exit_thread() syscalls.

LibThread is also ported to using LibPthread.

Libraries/LibPthread/pthread.cpp

@@ -0,0 +1,24 @@
+#include <AK/StdLibExtras.h>
+#include <pthread.h>
+#include <unistd.h>
+
+extern "C" {
+
+int pthread_create(pthread_t* thread, pthread_attr_t* attributes, void *(*start_routine)(void*), void* argument_to_start_routine)
+{
+    if (!thread)
+        return -EINVAL;
+    UNUSED_PARAM(attributes);
+    int rc = create_thread(start_routine, argument_to_start_routine);
+    if (rc < 0)
+        return rc;
+    *thread = rc;
+    return 0;
+}
+
+void pthread_exit(void* value_ptr)
+{
+    exit_thread(value_ptr);
+}
+
+}