better support cross-platform injection

_From [zhao...@google.com](https://code.google.com/u/106321947286816917100/) on June 02, 2013 22:50:03_

We now have -32/-64 option for cross-platform injection.
Can we have a automagic way to support it without users passing options?
xfer issue #147 , issue #1147 , and DrM-issue #1037 ( https://code.google.com/p/drmemory/issues/detail?id=1037 )

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1176_