Implement Zcb extension

[Timmmm]

This adds an implementation of the Zcb code size extension.

This fixes all of the comments on the previous PR #322 - and also adds flags to the emulator to enable the extension (it is disabled by default).

[github-actions]

Test Results

712 tests  ±0   712 ✅ ±0   0s ⏱️ ±0s
  6 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 910f90b. ± Comparison against base commit a2ffa85.

♻️ This comment has been updated with latest results.

[arichardson]

I have not checked the encodings are correct, but the code LGTM.
I'm not sure we actually need this command line flag (especially if you're not actually using it).

[Timmmm]

I have not checked the encodings are correct, but the code LGTM. I'm not sure we actually need this command line flag (especially if you're not actually using it).

I mainly added it for other people to use and to reduce the merge conflicts for us. I think it's worth keeping for when/if we eventually actually use riscv-config. Though I'm happy to be overruled if it would block this being merged.

[arichardson]

I have not checked the encodings are correct, but the code LGTM. I'm not sure we actually need this command line flag (especially if you're not actually using it).

I mainly added it for other people to use and to reduce the merge conflicts for us. I think it's worth keeping for when/if we eventually actually use riscv-config. Though I'm happy to be overruled if it would block this being merged.

I'm guessing a lot of this command line parsing will go away eventually, so if it makes things easier for you merging as is sounds good.

[billmcspadden-riscv]

LGTM