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

[Craig Topper via cfe-commits]

topperc wrote:

> Thanks!
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> 
> 
> If someone sets zba_zbb_zbs, should b be inferred?
> 
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> 

Yes, but it will break `-march=rv64i_zba_zbb_zbs -fno-integrated-as` with versions of the external assembler that don't know about B. We'll infer B and pass it to the assembler -march.

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