LibPthread: Partially implement pthread_cleanup_(push pop)

These are also supposed to run on cancellation, but we don't support
cancellation.

Userland/Libraries/LibPthread/pthread.cpp

@@ -9,6 +9,7 @@
 #include <AK/Debug.h>
 #include <AK/Format.h>
 #include <AK/StdLibExtras.h>
+#include <AK/Vector.h>
 #include <Kernel/API/Syscall.h>
 #include <LibSystem/syscall.h>
 #include <bits/pthread_integration.h>
@@ -37,6 +38,12 @@ static constexpr size_t highest_reasonable_stack_size = 8 * MiB; // That's the d
 __thread void* s_stack_location;
 __thread size_t s_stack_size;
 
+struct CleanupHandler {
+    void (*function)(void*) { nullptr };
+    void* argument { nullptr };
+};
+thread_local Vector<CleanupHandler> s_cleanup_handlers {};
+
 #define __RETURN_PTHREAD_ERROR(rc) \
     return ((rc) < 0 ? -(rc) : 0)
 
@@ -139,19 +146,23 @@ int pthread_create(pthread_t* thread, pthread_attr_t* attributes, void* (*start_
 // https://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_exit.html
 void pthread_exit(void* value_ptr)
 {
+    while (!s_cleanup_handlers.is_empty())
+        pthread_cleanup_pop(1);
     exit_thread(value_ptr, s_stack_location, s_stack_size);
 }
 
 // https://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_cleanup_push.html
-void pthread_cleanup_push([[maybe_unused]] void (*routine)(void*), [[maybe_unused]] void* arg)
+void pthread_cleanup_push(void (*routine)(void*), void* arg)
 {
-    TODO();
+    s_cleanup_handlers.empend(routine, arg);
 }
 
 // https://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_cleanup_pop.html
-void pthread_cleanup_pop([[maybe_unused]] int execute)
+void pthread_cleanup_pop(int execute)
 {
-    TODO();
+    auto entry = s_cleanup_handlers.take_last();
+    if (execute != 0)
+        entry.function(entry.argument);
 }
 
 // https://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_join.html