Fix clang build adding rpath to libstdc++

The binaries generated during the build process of clang are missing the
RPATH of the libstdc++.so library, which is provided by gcc libs.
Similarly, the clang binary itself also needs the rpath to the
libstdc++.so library path.

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>