[libc] Implement atexit et al

This commit joins the implementation of exit, _Exit, and abort into a
single translation unit, and also adds atexit, at_quick_exit, and
quick_exit. While this does go against the ideal of all libc functions
being in their own translation unit, their implementations are very
related, and so I think this makes sense.

src/libraries/libc/stdlib/_Exit.cpp

@@ -1,18 +0,0 @@
-/** \file _Exit.cpp
-  *
-  * This file is part of the C standard library for the jsix operating
-  * system.
-  *
-  * This Source Code Form is subject to the terms of the Mozilla Public
-  * License, v. 2.0. If a copy of the MPL was not distributed with this
-  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
-  */
-
-#include <stdlib.h>
-#include <j6/syscalls.h>
-
-void
-_Exit( int status )
-{
-    j6_process_exit(status);
-}

src/libraries/libc/stdlib/abort.cpp

@@ -1,18 +0,0 @@
-/** \file abort.cpp
-  *
-  * This file is part of the C standard library for the jsix operating
-  * system.
-  *
-  * This Source Code Form is subject to the terms of the Mozilla Public
-  * License, v. 2.0. If a copy of the MPL was not distributed with this
-  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
-  */
-
-#include <stdlib.h>
-#include <stdint.h>
-
-void
-abort()
-{
-    _Exit(INT32_MIN);
-}

src/libraries/libc/stdlib/exit.cpp

@@ -1,17 +1,85 @@
-/** \file exit.cpp
-  *
-  * This file is part of the C standard library for the jsix operating
-  * system.
-  *
-  * This Source Code Form is subject to the terms of the Mozilla Public
-  * License, v. 2.0. If a copy of the MPL was not distributed with this
-  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
-  */
-
 #include <stdlib.h>
+#include <j6/syscalls.h>
+
+using cxa_callback = void (*)(void *);
+using c_callback = void (*)(void);
+
+namespace {
+
+constexpr size_t max_atexit = 64;
+
+struct atexit_item
+{
+    cxa_callback cb;
+    void *arg;
+};
+
+size_t atexit_count = 0;
+atexit_item atexit_array[max_atexit];
+
+size_t quick_exit_count = 0;
+atexit_item quick_exit_array[max_atexit];
+
+[[noreturn]] inline void
+exit_with_callbacks(int status, atexit_item *cbs, size_t count)
+{
+    for (size_t i = count - 1; i < count; --i) {
+        atexit_item &item = cbs[i];
+        item.cb(item.arg);
+    }
+
+    _Exit(status);
+}
+
+void
+bare_callback(void *real_cb)
+{
+    reinterpret_cast<c_callback>(real_cb)();
+}
+
+} // namespace
+
+extern "C" int
+__cxa_atexit(cxa_callback cb, void *arg, void *dso)
+{
+    if (atexit_count == max_atexit) return -1;
+    atexit_array[atexit_count++] = {cb, arg};
+    return 0;
+}
+
+int
+atexit( void (*func)(void) )
+{
+    return __cxa_atexit(bare_callback, reinterpret_cast<void*>(func), nullptr);
+}
+
+int at_quick_exit( void (*func)(void) )
+{
+    if (quick_exit_count == max_atexit) return -1;
+    quick_exit_array[quick_exit_count++] = {&bare_callback, reinterpret_cast<void*>(func)};
+    return 0;
+}
 
 void
 exit(int status)
 {
-    _Exit(status);
+    exit_with_callbacks(status, atexit_array, atexit_count);
+}
+
+void
+quick_exit(int status)
+{
+    exit_with_callbacks(status, quick_exit_array, quick_exit_count);
+}
+
+void
+abort()
+{
+    _Exit(INT32_MIN);
+}
+
+void
+_Exit( int status )
+{
+    j6_process_exit(status);
 }