handle shared memory written externally

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on December 10, 2010 17:57:47_

PR 408535

I don't have a solution here other than being in the other process and
sending a message on each write to shared memory, or maybe having an
API to mark as shared and just not check that region (though regular
suppression files should work too). Xref all the IPC issues that
any system like this, or base DR, has.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=117_