[clang] [llvm] [RISCV] Add B extension (PR #76893)

[Alex Bradbury via llvm-commits]

asb wrote:

Thanks!

If someone sets zba_zbb_zbs, should b be inferred?

This patch needs an LLVM release note as well, and agreed we should await on a versioning decision.

https://github.com/llvm/llvm-project/pull/76893