Revert "llvm: only build x86 and riscv"

This reverts commit b17ec6fb1f.

derivations.nix

@@ -18,7 +18,7 @@ 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" "-DLLVM_TARGETS_TO_BUILD=X86"];
+    cmakeFlags = oa.cmakeFlags ++ ["-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=RISCV"];
   });
   rustc = (pkgs.rustc.overrideAttrs(oa: {
     name = "${oa.pname}-${oa.version}-riscv";