Signal handling in hybrid execution

_From [zhao...@google.com](https://code.google.com/u/106321947286816917100/) on August 05, 2013 12:37:39_

DynamoRIO was originally designed to run all program under DR all the time, so all the signal will go through the master signal handler for handling.

To support a process partially running under DR and partially running natively, we need adjust the signal handler to handle the case when a signal is delivered during the native execution.

Currently, it probably will cause assertion.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1242_