exit: annotate FUNC_ATTR_NORETURN functions #7954 (#7954)

This should fix a particular false positive from clang 5.0.0 scan-build,
which thinks that nlua_init() can continue after preserve_exit().

src/nvim/os_unix.c

@@ -133,7 +133,8 @@ void mch_free_acl(vim_acl_T aclent)
 }
 #endif
 
-void mch_exit(int r) FUNC_ATTR_NORETURN
+void mch_exit(int r)
+  FUNC_ATTR_NORETURN
 {
   exiting = true;