[libre-riscv-dev] Instruction sorta-prefixes for easier high-register access

[Luke Kenneth Casson Leighton]

btw jacob i'm not saying "no" to modified instructions (non-RV),
especially given that the priority is llvm, where yes we have to
maintain a custom version of that: gcc and binutils can be deferred.

what i'm saying is: if there's any other way (such as the implicit
0b00-prefix => scalar idea) i'd prefer that to be prioritised.

l.