Configure G++ to support the `-stdlib=` option

[cpplearner]

GCC recently added a -stdlib= option, which makes it easier to use libc++ with g++. But this option is only enabled when GCC is configured with the --with-gxx-libcxx-include-dir= option.

It would be nice if compiler-explorer could support this option.

[partouf]

Not exactly sure how we can make this work and also be practical for the website. I guess we could set it to clang-trunk for gcc trunk, but if this gets into a release it will be a bit less flexible than the --gcc-toolchain option the clangs have - unless they decide to include something similar.