Fix windows_nat_target::fake_create_process ptid

While working on Windows non-stop mode, I managed to introduce a bug
that led to fake_create_process being called.  That then resulted in
GDB crashes later on, because fake_create_process added a thread with
an incorrect ptid for this target.  It is putting dwThreadId in the
tid field of the ptid instead of on the lwp field.  This is fixed by
this patch.

Change-Id: Iaee5d2deaa57c501f7e6909f8ac242af9b183215