Add an ARM CI test

Tries to be as strict as possible.  Unfortunately Address Sanitizer
appears to be very slow on this platform at the moment, so has
been left out for now.  It would be good to add it later should
its performance improve.

.cirrus.yml

@@ -140,6 +140,33 @@ rocky_task:
   << : *COMPILE
   << : *TEST
 
+# Arm Linux
+arm_ubuntu_task:
+  name: ubuntu-arm
+  arm_container:
+    image: ubuntu:latest
+    cpu: 2
+    memory: 1G
+
+  environment:
+    LC_ALL: C
+    CIRRUS_CLONE_DEPTH: 1
+    DO_UNTRACKED_FILE_CHECK: yes
+    USE_CONFIG: yes
+    CFLAGS: -g -Wall -O3 -std=c99 -pedantic
+
+  # NB: we could consider building a docker image with these
+  # preinstalled and specifying that instead, to speed up testing.
+  install_script: |
+    apt-get update
+    apt-get install -y --no-install-suggests --no-install-recommends     \
+        ca-certificates clang libc-dev make git autoconf automake        \
+        zlib1g-dev libbz2-dev liblzma-dev libcurl4-gnutls-dev libssl-dev \
+        libdeflate-dev
+
+  << : *COMPILE
+  << : *TEST
+
 #--------------------------------------------------
 # Task: macOS builds