[compiler-rt] [RISCV][compiler-rt] drop __riscv_vendor_feature_bits (PR #126460)

[Kito Cheng via llvm-commits]

https://github.com/kito-cheng approved this pull request.

LGTM, and I think we need to backport this to LLVM 20?

https://github.com/llvm/llvm-project/pull/126460