Teach the assembler to recognise standard CSR names

[asb]

No description provided.

[dvc94ch]

What are your thoughts on how to do this? The options I see are either a custom operand parser or a new RegisterClass.

But in both cases you have made it uneasy, I presume on purpose... (CSR is already defined for Callee Saved Registers and the imm12 tests use the csr instructions even dough you could have selected from a variety of other ones)

[asb]

I haven't looked at this much yet. The CSR instructions actually are the only uimm12 instructions, which is why the assembler tests exercise this. The ability to use uimm12 integer operand needs to be preserved even once standard CSR names are recognised.