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

[Wang Pengcheng via cfe-commits]

wangpc-pp wrote:

I will hold this PR util B extension is ratified (I think it won't take too long). Or, at least after llvm 18 branch is created.

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