Support NetBSD Thread ID in lldb-server tests

Summary:
Native Thread ID is retrieved with _lwp_self() on NetBSD.

The returned value is of type int32_t, but for consistency with other Operating Systems cast it to uint64_t.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, labath, clayborg, emaste

Reviewed By: labath, clayborg

Subscribers: #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D30374

llvm-svn: 296360