Configure the dist-x86_64-linux builder to produce compiler documentation

Author: davidtwco

src/ci/docker/dist-x86_64-linux/Dockerfile

@@ -84,7 +84,8 @@ ENV HOSTS=x86_64-unknown-linux-gnu
 ENV RUST_CONFIGURE_ARGS \
       --enable-full-tools \
       --enable-sanitizers \
-      --enable-profiler
+      --enable-profiler \
+      --enable-compiler-docs
 ENV SCRIPT python2.7 ../x.py dist --host $HOSTS --target $HOSTS
 
 # This is the only builder which will create source tarballs