It looks like the whole mess1() (etc) macro system is going to fail on
64-bit ARM - I can't see how to patch that.  I can fix the local problems in
the Pd vanilla source but I don't know whether it's better to take out the
"mess()" macros altogether, or to take them out only for ARM64.  (The
former would be source incompatible but object compatible, but I'm thinking
source compatibilty with a non-working feature is maybe actually worse than
simply being source incompatible.)

I've gone ahead and git-pushed an attempted fix (changing mess1() etc, 
potentially source-incompatibly) but am open to other ideas how to fix this.