Regenerate encoding.h from riscv-opcodes (#848)

This will enable the Krypto changes in #846 to apply cleanly.

This removes the encodings for N-extension (user interrupts) which
were never implemented in Spike; also fixes the encoding of
sfence.w.inval and sfence.inval.ir, confirmed by @daniellustig:
https://github.com/riscv-software-src/riscv-isa-sim/pull/846#discussion_r741437419

This was generated from
https://github.com/riscv/riscv-opcodes/commit/70c120b5464d0a77491f07e5d66c4523d63a86a4
which I created to get the generated encoding.h to most closely match
what Spike already had; see https://github.com/riscv/riscv-opcodes/pull/87