Add Smdbltrp

[ved-rivos]

Spec: https://github.com/riscv/riscv-double-trap
Tests: riscv-non-isa/riscv-arch-test#438

[ved-rivos]

This PR has a encoding.h change which is not merged upstream in riscv-opcodes yet. I will refresh it once the riscv-opcodes PR riscv/riscv-opcodes#257 gets merged.

[aswaterman]

I merged the riscv-opcodes PR using a merge commit. The SHA hash is still valid, so there is no need to refresh encoding.h.

Please squash the two commits into a single commit, though.

I’ll perform a code review next week.

[aswaterman]

I didn't think about this one too deeply, but it looks good to me based on a cursory review. Thanks!

[aswaterman]

@ved-rivos please fix the new conflicts and rebase/squash, then ping me to merge it.

[ved-rivos]

@aswaterman thanks. The conflicts have been resolved.

[ved-rivos]

@aswaterman - ptal if further changes are needed.

[aswaterman]

@ved-rivos thanks for the ping; I must've missed your previous message.