native_exec gateway bbs should support unlinking

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on August 02, 2013 16:40:12_

DR needs to be able to unlink any fragment (for cache consistency using relaxed consistency models, for signal delivery delay bounds, etc.). Thus we want to be able to unlink a native_exec gateway bb. And once we have that capability, having it be the last bb in a trace will also work, b/c while building the trace we'll unlink and come back to DR and finalize the trace with the final bb linked so that future executions will go straight native.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1239_