Rollup merge of #100528 - tux3:riscv-bitmanip-features, r=davidtwco

Support 1st group of RISC-V Bitmanip backend target features

These target features use the same names as LLVM and `is_riscv_feature_detected!`, they are:
- zba (address generation instructions)
- zbb (basic bit manipulation)
- zbc (carry-less multiplication)
- zbs (single-bit manipulation)

The extension is frozen and ratified, and I don't think we should expect LLVM to change those feature names in the future.
For reference, the specification for the B extension can be found here: https://github.com/riscv/riscv-bitmanip/releases/download/1.0.0/bitmanip-1.0.0-38-g865e7a7.pdf)

On my current project, I see a 7.6% reduction in binary size with these features on, so I have some incentive to try to silence the "unknown feature" warning from `-Ctarget-feature` =)

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