Use KSTART_DC in dispatch()

Profiling shows that get_thread_private_dcontext() takes 1% of the
time on -native_exec DumpRenderTree.  Currently -native_exec ends up
in dispatch far more often than we would if everything were linked
together in the cache.

Review URL: https://codereview.appspot.com/7376057

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1944 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6
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