i#1077: Avoid interpreting back_from_native

This adds a check to dispatch() to see if we're about to interpret one
of the back_from_native return stubs.  If we hit one of these stubs, we
pop the appropriate app return address from the native return stack and
continue interpreting at that PC.

Review URL: https://codereview.appspot.com/7407048

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1954 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6
Merge branch 'master' into native-exec-interp-native