Define LIBCXX_USE_COMPILER_RT while building libcxx

This makes a lot more libcxx cmake configure tests work as intended,
which makes a lot more -W* options added, resulting in much more build
warnings.

Dockerfile

@@ -221,6 +221,7 @@ RUN cd libcxx && \
             -DCMAKE_CXX_COMPILER_WORKS=TRUE \
             -DCMAKE_AR=$TOOLCHAIN_PREFIX/bin/$AR \
             -DCMAKE_RANLIB=$TOOLCHAIN_PREFIX/bin/$RANLIB \
+            -DLIBCXX_USE_COMPILER_RT=ON \
             -DLIBCXX_INSTALL_HEADERS=ON \
             -DLIBCXX_ENABLE_EXCEPTIONS=$EXCEPTIONS \
             -DLIBCXX_ENABLE_THREADS=OFF \