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

src/ds/aba.h

@@ -99,9 +99,11 @@ namespace snmalloc
           (__int64)value,
           (__int64*)&old);
 #  else
-#    if defined(__GNUC__) && !defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16)
+#    if defined(__GNUC__) && defined(SNMALLOC_VA_BITS_64) && \
+      !defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16)
 #error You must compile with -mcx16 to enable 16-byte atomic compare and swap.
 #    endif
+
         Linked xchg{value, old.aba + 1};
         std::atomic<Linked>& addr = parent->linked;