Add B extension

[ved-rivos]

Description

Add unratified B extension. B extension includes Zba, Zbb, and Zbs.

Related Issues

N/A

Update to/for Ratified/Unratified Extensions

• Ratified
• Unratified

List Extensions

https://github.com/riscv/riscv-b

Mandatory Checklist:

• Make sure you have updated the versions in setup.cfg and riscv_config/__init__.py. Refer to CONTRIBUTING.rst file for further information.
• Make sure to have created a suitable entry in the CHANGELOG.md.

[tux3]

I believe B has now been ratified, so this change would be even more welcome!