feat(toolchain): add recipe to build riscv64 toolchain

Signed-off-by: Diogo Costa <diogoandreveigacosta@gmail.com>

pkgs/toolchains/riscv64-unknown-elf-10.2.nix

@@ -0,0 +1,46 @@
+# SPDX-License-Identifier: Apache-2.0
+# Copyright (c) Bao Project and Contributors. All rights reserved.
+
+{ lib
+, stdenv
+, fetchurl
+, ncurses5
+, python38
+, zlib
+}:
+
+stdenv.mkDerivation rec {
+  pname = "riscv64-unknown-elf";
+  version = "1.0.0";
+  arch = "x86_64";
+
+  src = fetchurl {
+    url = "https://static.dev.sifive.com/dev-tools/freedom-tools/v2020.12/riscv64-unknown-elf-toolchain-10.2.0-2020.12.8-${arch}-linux-ubuntu14.tar.gz";
+    sha256 = "sha256-vYVyQrfGSxqYxWs1vIAYZKxCBdA/ODxwEKkMXQu5VJo=";
+  };
+
+  nativeBuildInputs = [ zlib ]; #build time dependencies
+
+
+  dontConfigure = true;
+  dontBuild = true;
+  dontPatchELF = true;
+  dontStrip = true;
+
+  installPhase = ''
+    mkdir -p $out
+    cp -r * $out
+  '';
+
+  # Package -> put the binaries with nix store paths
+  preFixup = ''
+    find $out -type f | while read f; do
+      patchelf "$f" > /dev/null 2>&1 || continue
+      patchelf --set-interpreter $(cat ${stdenv.cc}/nix-support/dynamic-linker) "$f" || true
+      patchelf --set-rpath ${lib.makeLibraryPath [ "$out" stdenv.cc.cc ncurses5 python38 zlib]} "$f" || true
+    done
+  '';
+
+}
+
+