There are many peculiarites of the ARM architecture that require work:
condition codes, new instructions, new instruction arg counts, and more.
Rewrite the parser to do a cleaner job, flowing left to right through the
sequence of elements of an operand.
Add ARM to arch.
Add ARM-specific details to the arch in a new file, internal/arch/arm.
These are probably better kept away from the "portable" asm. However
there are some pieces, like MRC, that are hard to disentangle. They
can be cleaned up later.