instrument_post_syscall() should be called after post_system_call() processes the real result

From [email protected] on February 11, 2009 13:53:44

I threw in the syscall API too quickly it seems: setting the mcontext/result post-syscall should be after DR handles the syscall and should be considered only a cosmetic result for fooling the app.

xref PR 207947 on syscall API feature

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1