i#978: Use a stack of native retaddrs to handle nested native calls

This assumes the app is only every unwinding and never re-entering
bypassed frames.

Adds a test of an app that uses longjmp to unwind past a swapped return
address.

Review URL: https://codereview.appspot.com/7345044

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1945 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6
Merge branch 'master' into native-exec-retstack