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

[Craig Topper via cfe-commits]

topperc wrote:

> I would suggest set it as 0.1 rather than 1.0, and I gonna to ask Ved to add version info as well...

Then also needs to move behind -menable-experimental-extensions.

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