[clang] [llvm] [RFC][RISCV] Use the 'B' extension in RISC-V profile definitions (PR #113942)

[Alex Bradbury via cfe-commits]

asb wrote:

> I think we shouldn't do this for RVA22, so as not to break existing users of that profile, who may have a toolchain that doesn't support B.
> 
> This change makes sense to me for the RV*23 profiles, especially since your change to RVM23 has been accepted.

For RVA22, I had a look at the commit history for extension support in binutils. From what I can see (@kito-cheng please correct me if I'm wrong):
* zic64b support was only committed in 25f05199bb7e35820c23e802424484accb7936b1 in July 2024
* B support was committed in c144f638337944101131d9fe6de4ab908f6d4c2d in May 2024

So given we emit zic64b anyway (as it has always been in the RVA22 spec), I think we probably do have freedom to add in B here without breaking binutils that would have previously successfully assembled our rva22u64 output. 

https://github.com/llvm/llvm-project/pull/113942