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

[Kito Cheng via cfe-commits]

kito-cheng 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] https://github.com/riscv/riscv-isa-manual/commit/cdb25859bcce5a6edbdc34914c1f86f079b3fddd

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