[llvm] [RISCV] Add groupid/bitmask for RISC-V extension (PR #94440)

[Craig Topper via llvm-commits]

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@@ -37,6 +37,15 @@ class RISCVExtension<string name, int major, int minor, string desc,
   bit Experimental = false;
 }
 
+// The groupID/bitmask of RISCVExtension is used to retrieve a specific bit value 
+// from __riscv_feature_bit based on the groupID and bitmask.
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topperc wrote:

It's called `__riscv_feature_bits` with an `s` on the end in the C API right?

https://github.com/llvm/llvm-project/pull/94440