Aconv is the pretty-printer for instruction opcodes like AMOVQ.
There was one for each architecture.
Make the space of A names have a different region for each architecture,
much as we did for the registers, so a single global Aconv function can
do the work. Each architecture registers its region as a slice of names
at a given offset.
The global names like CALL and JMP are now defined only once.
The A values are used for indexing tables, so make it easy to do the
indexing by making the offset maskable.
Remove a bunch of now-duplicated architecture-specific code.