Add runtime.exit

base/runtime/os_specific_freestanding.odin

@@ -6,3 +6,7 @@ package runtime
 _stderr_write :: proc "contextless" (data: []byte) -> (int, _OS_Errno) {
 	return 0, -1
 }
+
+_exit :: proc "contextless" (code: int) -> ! {
+	trap()
+}