i#978: Take control back from native ELF modules at cross-module PLT calls

This behavior is controlled by -native_exec_retakeover.  When returning to a
native module with this option set, we trust that it will return control back
to DR.  We generate special bbs that jump directly back into the module on such
returns.

Only one level of native execution is currently supported.  We assert on nested
calls into native modules.

Review URL: https://codereview.appspot.com/7324044

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1909 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6
Merge branch 'master' into native-exec-takeover