libstdcxx: don’t set stdlib automatically

We don’t need to set -stdlib=libstdc++. This only works on Clang so it
is not good to set it globally. In addition, Clang knows to use
libstdc++ on Linux by default if no stdlib is set:

https://github.com/llvm-mirror/clang/blob/324f918438715b4a0d024af5930628c1674f4fcd/lib/Driver/ToolChains/Linux.cpp#L456

It’s a good policy to just leave off stdlib for now.

Fixes #29877.