Test the c-system feature in CI as well

ci/docker/aarch64-unknown-linux-gnu/Dockerfile

@@ -1,10 +1,13 @@
 FROM ubuntu:18.04
-RUN apt-get update && \
+RUN dpkg --add-architecture arm64 && \
+    apt-get update && \
     apt-get install -y --no-install-recommends \
     gcc libc6-dev ca-certificates \
     gcc-aarch64-linux-gnu libc6-dev-arm64-cross \
+    clang libclang-common-6.0-dev:arm64 \
     qemu-user-static
 ENV CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER=aarch64-linux-gnu-gcc \
     CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_RUNNER=qemu-aarch64-static \
     QEMU_LD_PREFIX=/usr/aarch64-linux-gnu \
+    CLANG=clang \
     RUST_TEST_THREADS=1

ci/docker/armv7-unknown-linux-gnueabihf/Dockerfile

@@ -1,9 +1,13 @@
 FROM ubuntu:18.04
-RUN apt-get update && \
+RUN dpkg --add-architecture armhf && \
+    apt-get update && \
     apt-get install -y --no-install-recommends \
     gcc libc6-dev ca-certificates \
-    gcc-arm-linux-gnueabihf libc6-dev-armhf-cross qemu-user-static
+    gcc-arm-linux-gnueabihf libc6-dev-armhf-cross \
+    clang libclang-common-6.0-dev:armhf \
+    qemu-user-static
 ENV CARGO_TARGET_ARMV7_UNKNOWN_LINUX_GNUEABIHF_LINKER=arm-linux-gnueabihf-gcc \
     CARGO_TARGET_ARMV7_UNKNOWN_LINUX_GNUEABIHF_RUNNER=qemu-arm-static \
     QEMU_LD_PREFIX=/usr/arm-linux-gnueabihf \
+    CLANG=clang \
     RUST_TEST_THREADS=1

ci/docker/i686-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,7 @@
 FROM ubuntu:18.04
-RUN apt-get update && \
+RUN dpkg --add-architecture i386 && \
+    apt-get update && \
     apt-get install -y --no-install-recommends \
-    gcc-multilib libc6-dev ca-certificates
+    gcc-multilib libc6-dev ca-certificates \
+    clang libclang-common-6.0-dev:i386
+ENV CLANG=clang

ci/docker/powerpc64le-unknown-linux-gnu/Dockerfile

@@ -1,13 +1,16 @@
 FROM ubuntu:18.04
 
-RUN apt-get update && \
+RUN dpkg --add-architecture ppc64el && \
+    apt-get update && \
     apt-get install -y --no-install-recommends \
-    gcc libc6-dev qemu-user-static ca-certificates \
+    gcc libc6-dev ca-certificates \
     gcc-powerpc64le-linux-gnu libc6-dev-ppc64el-cross \
-    qemu-system-ppc
+    clang libclang-common-6.0-dev:ppc64el \
+    qemu-user-static qemu-system-ppc
 
 ENV CARGO_TARGET_POWERPC64LE_UNKNOWN_LINUX_GNU_LINKER=powerpc64le-linux-gnu-gcc \
     CARGO_TARGET_POWERPC64LE_UNKNOWN_LINUX_GNU_RUNNER=qemu-ppc64le-static \
     QEMU_CPU=POWER8 \
     QEMU_LD_PREFIX=/usr/powerpc64le-linux-gnu \
+    CLANG=clang \
     RUST_TEST_THREADS=1

ci/docker/x86_64-unknown-linux-gnu/Dockerfile

@@ -1,4 +1,4 @@
 FROM ubuntu:18.04
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
-    gcc libc6-dev ca-certificates
+    gcc libc6-dev ca-certificates clang

ci/run.sh

@@ -18,6 +18,10 @@ cargo build --target $1
 cargo build --target $1 --release
 cargo build --target $1 --features c
 cargo build --target $1 --release --features c
+if [ -n "$CLANG" -o -n "$LLVM_CONFIG" ]; then
+cargo build --target $1 --features c-system
+cargo build --target $1 --release --features c-system
+fi
 
 PREFIX=$(echo $1 | sed -e 's/unknown-//')-
 case $1 in
@@ -77,6 +81,10 @@ RUSTFLAGS="-C debug-assertions=no" $build_intrinsics
 RUSTFLAGS="-C debug-assertions=no" $build_intrinsics --release
 RUSTFLAGS="-C debug-assertions=no" $build_intrinsics --features c
 RUSTFLAGS="-C debug-assertions=no" $build_intrinsics --features c --release
+if [ -n "$CLANG" -o -n "$LLVM_CONFIG" ]; then
+RUSTFLAGS="-C debug-assertions=no" $build_intrinsics --features c-system
+RUSTFLAGS="-C debug-assertions=no" $build_intrinsics --features c-system --release
+fi
 
 # Verify that there are no undefined symbols to `panic` within our
 # implementations