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

[Alex Bradbury via cfe-commits]

asb wrote:

> It passed public review[1] and merged into riscv-isa-manual[2], so I think it's time to mark it as 1.0 and moving forward :)
> 
> [1] https://groups.google.com/a/groups.riscv.org/g/isa-dev/c/KetVUCQkfK4/m/Y3Dbd2pvAAAJ?utm_medium=email&utm_source=footer [2] [riscv/riscv-isa-manual at cdb2585](https://github.com/riscv/riscv-isa-manual/commit/cdb25859bcce5a6edbdc34914c1f86f079b3fddd)

That's probably true - though historically I've been using https://wiki.riscv.org/display/HOME/Recently+Ratified+Extensions to determine if a spec is ratified or not, now moved to https://wiki.riscv.org/display/HOME/Ratified+Extensions seemingly. B doesn't seem to be listed yet. @jjscheel - is it just not added to the wiki page yet, or is it technically not yet ratified?

(I know it's really a trivial "extension" so might be a special case)

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