Remove the code forcing -stdlib=libstdc++ on NetBSD in getBuildFlags()
method. NetBSD uses libc++ everywhere else, and using libstdc++ here
causes lang/cpp/dynamic-value to fail to build.
NB: I have no clue what getBuildFlags() is supposed to do, and why it's used in the few tests it's used in, so the change can be entirely wrong. However, tests pass for me with it, and it fixes missing symbols I've had when building with libstdc++.