add c11 quick_exit and at_quick_exit functions

include/stdlib.h

@@ -43,6 +43,8 @@ void abort (void);
 int atexit (void (*) (void));
 void exit (int);
 void _Exit (int);
+int at_quick_exit (void (*) (void));
+void quick_exit (int);
 
 char *getenv (const char *);
 

src/exit/at_quick_exit.c

+#include <stdlib.h>
+#include "libc.h"
+
+#define COUNT 32
+
+static void (*funcs[COUNT])(void);
+static int count;
+static int lock[2];
+
+void __funcs_on_quick_exit()
+{
+	void (*func)(void);
+	LOCK(lock);
+	while (count > 0) {
+		func = funcs[--count];
+		UNLOCK(lock);
+		func();
+		LOCK(lock);
+	}
+}
+
+int at_quick_exit(void (*func)(void))
+{
+	if (count == 32) return -1;
+	LOCK(lock);
+	funcs[count++] = func;
+	UNLOCK(lock);
+	return 0;
+}

src/exit/quick_exit.c

+#include <stdlib.h>
+#include "syscall.h"
+#include "atomic.h"
+#include "libc.h"
+
+static void dummy() { }
+weak_alias(dummy, __funcs_on_quick_exit);
+
+void quick_exit(int code)
+{
+	static int lock;
+	while (a_swap(&lock, 1)) __syscall(SYS_pause);
+	__funcs_on_quick_exit();
+	_Exit(code);
+}