[libre-riscv-dev] uniform instruction format

[Luke Kenneth Casson Leighton]

Someone named EricP spotted something, suggested an idea that was basically
equivalent to uepc and a bit more.

This caused me to review the handling of STATE in user mode traps and I
realised we need STATE, ueSTATE, seSTATE and meSTATE.

These to be swapped around a bit on traps, including u mode ones.

It actually reduces the number of CSRs, as an M Mode S Mode VL and MVL and
SUBVL are no longer needed.

Cool.



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