xa 2.3.11

I've updated xa, André Fachat's venerable 6502 cross-assembler, to version 2.3.11. This contains miscellaneous bug fixes and a compilation fix for gcc 10. As usual it is tested on pretty much all of my Un*x systems here including AIX, Mac OS X (PowerPC and Intel), NetBSD/mac68k and Linux/ppc64le. This will probably be the last in the venerable 2.3 series before 2.4, in which we may do some compatibility breaks and jettison a couple long-deprecated options. More to come on that.