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

llvm-svn: 348873