Make it possible to provide special (linker) flags for the thread tests.
Use it to build & link against libpthread on NetBSD for tests iff they
are testing the thread interface.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@180942 91177308-0d34-0410-b5e6-96231b3b80d8