mark riscv llvm

derivations.nix

@@ -17,6 +17,7 @@ rec {
 
   binutils-riscv = pkgs.callPackage ./compilers/binutils.nix { platform = "riscv32"; };
   llvm_7 = pkgs.llvm_7.overrideAttrs(oa: {
+    name = oa.name + "-riscv";
     cmakeFlags = oa.cmakeFlags ++ ["-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=RISCV"];
   });
   rustc = (pkgs.rustc.overrideAttrs(oa: {