add _Noreturn function attribute, with fallback for pre-C11 GNUC

src/exit/_Exit.c

@@ -1,7 +1,7 @@
 #include <stdlib.h>
 #include "syscall.h"
 
-void _Exit(int ec)
+_Noreturn void _Exit(int ec)
 {
 	__syscall(SYS_exit_group, ec);
 	__syscall(SYS_exit, ec);

src/exit/abort.c

@@ -2,7 +2,7 @@
 #include <signal.h>
 #include "syscall.h"
 
-void abort(void)
+_Noreturn void abort(void)
 {
 	raise(SIGABRT);
 	raise(SIGKILL);

src/exit/exit.c

@@ -14,7 +14,7 @@ weak_alias(dummy, __funcs_on_exit);
 weak_alias(dummy, __flush_on_exit);
 weak_alias(dummy, __seek_on_exit);
 
-void exit(int code)
+_Noreturn void exit(int code)
 {
 	static int lock;
 

src/exit/quick_exit.c

@@ -6,7 +6,7 @@
 static void dummy() { }
 weak_alias(dummy, __funcs_on_quick_exit);
 
-void quick_exit(int code)
+_Noreturn void quick_exit(int code)
 {
 	static int lock;
 	while (a_swap(&lock, 1)) __syscall(SYS_pause);