Add NetBSD support in Host::GetCurrentThreadID

Summary:
To retrieve the native thread ID there must be called _lwp_self().

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, clayborg, emaste, labath

Reviewed By: joerg, clayborg, labath

Subscribers: #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D29264