xa 2.3.12

I've updated xa, André Fachat's venerable 6502 cross-assembler, to version 2.3.12. This contains a bug fix for a regression in 65816 mode which I'd meant to release earlier but got sidetracked on (thanks Samuel Falvo for the nice test case, which is also incorporated into the suite). As with prior versions it is tested on pretty much all of my Un*x-alike systems here including AIX, Mac OS X (PowerPC, Intel and Apple Silicon), NetBSD/mac68k and Linux/ppc64le. I said this before for 2.3.11 but one more time for the record: this will probably be the last in the exceptionally long-lived 2.3 series before 2.4, which as I keep warning you will definitely have some minor compatibility breaks and jettison a couple long-deprecated options and syntaxes (but will have some new features to make up for it). Again, more to come on that.