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

[Wang Pengcheng via cfe-commits]

wangpc-pp 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.

Yes, this is why I set version to 1.0.
All implied extensions are ratified but when we want to use `b` to reduce some character in -march, we need to specify `-menable-experimental-extensions` instead.
(I hope B extension can be ratified soon...)

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