build-gcc: Create missing cc symlink

Author: tesuji

src/ci/docker/dist-x86_64-linux/build-gcc.sh

@@ -32,6 +32,7 @@ hide_output ../gcc-$GCC/configure \
     --enable-languages=c,c++
 hide_output make -j10
 hide_output make install
+ln -s gcc /rustroot/bin/cc
 
 cd ..
 rm -rf gcc-build