riscv64: add support for zbc instructions

[lxq015]

Add support for the disassembly of Zbc instructions and test cases.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR (HEAD: 77c297c) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/arch/+/735880.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
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  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
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• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
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• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.