llgs: add proper exec support for Linux.

This change:
* properly captures execs in NativeProcessLinux.
* clears out all non-main-thread thread metadata in NativeProcessLinux on exec.
* adds a DidExec() method to the NativeProcessProtocol delegate.
* clears out the auxv data cache when we exec (on Linux).

This is a small part of the llgs for local Linux debugging work going on here:
https://github.com/tfiala/lldb/tree/dev-llgs-local

I'm breaking it into small patches.