Implement __kmp_gettid() for NetBSD

Summary: _lwp_self() returns current Thread Id in a numeric version on NetBSD.

Reviewers: joerg, mgorny, #openmp

Reviewed By: mgorny

Subscribers: llvm-commits, openmp-commits, #openmp

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D55497

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@348873 91177308-0d34-0410-b5e6-96231b3b80d8