[user32] Add DECLSPEC_NOINLINE, use it on call_hook_proc()

[Qian Hong]

Hi Dan,

Thanks for working on it! It is really an annoying bug, however, is
adding DECLSPEC_NOINLINE a workaround or a real fix? If it is only a
workaround, would that hide some real bug behind it? Or is that
possible that there is a gcc bug here that we want to report to gcc?

Thanks for your time!