ci: Bring back ninja for dist builders

The primary reason for this is that make can result in a substantial
under utilization of parallelism, mostly due to the submake structure
preventing good dependency tracking and scheduling.

In f758c7b2a7 (Debian 6 doesn't have ninja, so use make for the dist builds)
llvm.ninja was disabled due to lack of distro package. This is no longer the
case with the CentOS 7 base, so bring ninja back for a performance boost.

src/ci/docker/host-x86_64/dist-i686-linux/Dockerfile

@@ -23,6 +23,7 @@ RUN yum upgrade -y && \
       libstdc++-devel.x86_64 \
       make \
       ncurses-devel \
+      ninja-build \
       openssl-devel \
       patch \
       perl \
@@ -64,7 +65,6 @@ ENV RUST_CONFIGURE_ARGS \
       --enable-profiler \
       --set target.i686-unknown-linux-gnu.linker=clang \
       --build=i686-unknown-linux-gnu \
-      --set llvm.ninja=false \
       --set rust.jemalloc
 ENV SCRIPT python3 ../x.py dist --build $HOSTS --host $HOSTS --target $HOSTS
 ENV CARGO_TARGET_I686_UNKNOWN_LINUX_GNU_LINKER=clang

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

@@ -23,6 +23,7 @@ RUN yum upgrade -y && \
       libstdc++-devel.x86_64 \
       make \
       ncurses-devel \
+      ninja-build \
       openssl-devel \
       patch \
       perl \
@@ -76,7 +77,6 @@ ENV RUST_CONFIGURE_ARGS \
       --set target.x86_64-unknown-linux-gnu.ar=/rustroot/bin/llvm-ar \
       --set target.x86_64-unknown-linux-gnu.ranlib=/rustroot/bin/llvm-ranlib \
       --set llvm.thin-lto=true \
-      --set llvm.ninja=false \
       --set rust.jemalloc \
       --set rust.use-lld=true
 ENV SCRIPT ../src/ci/pgo.sh python3 ../x.py dist \