Don't require 16-byte CAS on x86. (#173)

* Don't require 16-byte CAS on x86.

We only need to CAS 2 pointers, which is always possible on x86.

* Fix the way the Image variable is referenced.

* Replace failOnStderr by `set -eo pipefail` on Linux.

The x86 image doesn't have clangformat, which causes cmake to print a
warning on the stderr. Exit codes should be enough to detect failure.

* Use x86 images from snmallocciteam.

* clang-format

ci/linux_x86

@@ -0,0 +1,7 @@
+FROM multiarch/ubuntu-core:x86-bionic
+
+WORKDIR /src
+
+RUN apt update \
+    && apt install --no-install-recommends -y ninja-build clang++-9 cmake \
+    && apt -y clean