Replace Arm GCC to a more recent version

The version included in Ubuntu 16.04 repositories in the dist-various-1
docker, Arm GCC version 4.9, does not support the new Armv8-M
architecture.
This commit adds the team-gcc-arm-embedded PPA to get through APT a
newer version of Arm GCC.

src/ci/docker/dist-various-1/Dockerfile

@@ -21,12 +21,18 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   patch \
   libssl-dev \
   pkg-config \
-  gcc-arm-none-eabi \
   libnewlib-arm-none-eabi \
-  qemu-system-arm
+  qemu-system-arm \
+# software-properties-common for the add-apt-repository command
+  software-properties-common
 
 WORKDIR /build
 
+# Use the team-gcc-arm-embedded PPA for a newer version of Arm GCC
+RUN add-apt-repository ppa:team-gcc-arm-embedded/ppa && \
+    apt-get update && \
+    apt-get install -y --no-install-recommends gcc-arm-embedded
+
 COPY dist-various-1/build-rumprun.sh /build
 RUN ./build-rumprun.sh