Create a weak pthread_create reference on NetBSD to not force a
dependency on libpthread for code that doesn't use threads itself.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@182161 91177308-0d34-0410-b5e6-96231b3b80d8
Add NetBSD support.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@182162 91177308-0d34-0410-b5e6-96231b3b80d8