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` =)

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -227,6 +227,10 @@ const RISCV_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
     ("zhinxmin", Some(sym::riscv_target_feature)),
     ("zfh", Some(sym::riscv_target_feature)),
     ("zfhmin", Some(sym::riscv_target_feature)),
+    ("zba", Some(sym::riscv_target_feature)),
+    ("zbb", Some(sym::riscv_target_feature)),
+    ("zbc", Some(sym::riscv_target_feature)),
+    ("zbs", Some(sym::riscv_target_feature)),
     ("zbkb", Some(sym::riscv_target_feature)),
     ("zbkc", Some(sym::riscv_target_feature)),
     ("zbkx", Some(sym::riscv_target_feature)),