LibC: Support exit time destructors per the Itanium C++ ABI

Implement __cxa_atexit and __cxa_finalize per the Itanium spec,
and convert stdlib's atexit and exit() to to call them instead of
a custom 'C-only' atexit implementation.

Libraries/LibC/crt0.cpp

@@ -49,8 +49,49 @@ int _start(int argc, char** argv, char** env)
     ASSERT_NOT_REACHED();
 }
 
-void __cxa_atexit()
+typedef void (*AtExitFunction)(void *);
+
+struct __exit_entry
+{
+    AtExitFunction method;
+    void* parameter;
+    void* dso_handle;
+    bool has_been_called;
+};
+
+static __exit_entry __exit_entries[1024]{};
+static int __exit_entry_count = 0;
+
+int __cxa_atexit(AtExitFunction exit_function, void *parameter, void *dso_handle)
+{
+    if (__exit_entry_count >= 1024)
+        return -1;
+
+    __exit_entries[__exit_entry_count++] = { exit_function, parameter, dso_handle, false};
+
+    return 0;
+}
+
+void __cxa_finalize(void* dso_handle)
 {
+    // From the itanium abi, https://itanium-cxx-abi.github.io/cxx-abi/abi.html#dso-dtor-runtime-api
+    //
+    // When __cxa_finalize(d) is called, it should walk the termination function list, calling each in turn
+    // if d matches __dso_handle for the termination function entry. If d == NULL, it should call all of them.
+    // Multiple calls to __cxa_finalize shall not result in calling termination function entries multiple times;
+    // the implementation may either remove entries or mark them finished.
+
+    int entry_index = __exit_entry_count;
+
+    while (--entry_index >= 0)
+    {
+        auto& exit_entry = __exit_entries[entry_index];
+        bool needs_calling = !exit_entry.has_been_called && (!dso_handle || dso_handle == exit_entry.dso_handle);
+        if (needs_calling) {
+            exit_entry.method(exit_entry.parameter);
+            exit_entry.has_been_called = true;
+        }
+    }
 }
 
 extern u32 __stack_chk_guard;

Libraries/LibC/stdlib.cpp

@@ -106,14 +106,13 @@ static inline T strtol_impl(const char* nptr, char** endptr, int base)
 
 extern "C" {
 
-typedef void (*__atexit_handler)();
-static int __atexit_handler_count = 0;
-static __atexit_handler __atexit_handlers[32];
+// Itanium C++ ABI methods defined in crt0.cpp 
+extern int __cxa_atexit(void (*function)(void*), void* paramter, void* dso_handle);
+extern void __cxa_finalize(void* dso_handle);
 
 void exit(int status)
 {
-    for (int i = 0; i < __atexit_handler_count; ++i)
-        __atexit_handlers[i]();
+    __cxa_finalize(nullptr);
     extern void _fini();
     _fini();
     fflush(stdout);
@@ -122,11 +121,14 @@ void exit(int status)
     ASSERT_NOT_REACHED();
 }
 
+static void __atexit_to_cxa_atexit(void* handler)
+{
+    reinterpret_cast<void (*)()>(handler)();
+}
+
 int atexit(void (*handler)())
 {
-    ASSERT(__atexit_handler_count < 32);
-    __atexit_handlers[__atexit_handler_count++] = handler;
-    return 0;
+    return __cxa_atexit(__atexit_to_cxa_atexit, (void*)handler, nullptr);
 }
 
 void abort()