Open questions for architecture review

[arichardson]

This is a tracking issue for open questions that need to be addressed by architecture review:

• The extension names are currently not legal and are placeholders while we await official names
• Opcode allocation needs to be finalized (see also Add CHERI CSRs and encodings riscv-opcodes#328)
• Exception code reporting: We need to report both a fault address (xtval) and an exception "sub-code". Should this auxilliary exception information be reported in _x_tval2 as is currently the case or would it be cleaner to reserve the high 16 bits of _x_cause (see Swap roles of xtval and xtval2 #379 (review))?
• related: Should we be using the software check exception code instead of 0x1c? (Use software-check exception? #60)

Please let me know if I missed any issues here and I'll update this.