[lldb] [lldbtest] Fix getBuildFlags() not to use libstdc++ on NetBSD

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.

Differential Revision: https://reviews.llvm.org/D58871

llvm-svn: 355273