[lldb] [Process/FreeBSD] Set current thread ID on events

Set the current thread ID to the thread where an event happened.
As a result, e.g. when a signal is delivered to a thread other than
the first one, the respective T packet refers to the signaled thread
rather than the first thread (with no stop reason).  While this doesn't
strictly make a difference to the LLDB client, it is the expected
behavior.

Differential Revision: https://reviews.llvm.org/D117103