builtin: implement an at_exit(cb) wrapper for C.atexit (part 1) (#21254)

vlib/builtin/builtin.c.v

@@ -32,6 +32,23 @@ pub fn exit(code int) {
 	C.exit(code)
 }
 
+// at_exit registers a fn callback, that will be called at normal process termination.
+// It returns an error, if the registration was not successful.
+// The registered callback functions, will be called either via exit/1,
+// or via return from the main program, in the reverse order of their registration.
+// The same fn may be registered multiple times.
+// Each callback fn will called once for each registration.
+pub fn at_exit(cb FnExitCb) ! {
+	$if freestanding {
+		return error('at_exit not implemented with -freestanding')
+	} $else {
+		res := C.atexit(cb)
+		if res != 0 {
+			return error_with_code('at_exit failed', res)
+		}
+	}
+}
+
 // panic_debug private function that V uses for panics, -cg/-g is passed
 // recent versions of tcc print nicer backtraces automatically
 // Note: the duplication here is because tcc_backtrace should be called directly