[clang] [llvm] [RISC-V] Add CSR read/write builtins (PR #85091)

[Nemanja Ivanovic via cfe-commits]

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@@ -20,6 +20,12 @@ class RISCVBuiltin<string prototype, string features = ""> : TargetBuiltin {
 
 let Attributes = [NoThrow, Const] in {
 //===----------------------------------------------------------------------===//
+// Zicsr extension.
+//===----------------------------------------------------------------------===//
+def csrr : RISCVBuiltin<"unsigned long int(unsigned long int)", "zicsr">;
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nemanjai wrote:

I can certainly change it to `size_t`. And I'll move them out of the section marked with `Const`.

https://github.com/llvm/llvm-project/pull/85091