build: initial support for building nemu, am-kernels with nix

nemu/default.nix

@@ -0,0 +1,49 @@
+{ pkgs,
+  lib,
+  stdenv
+}:
+
+stdenv.mkDerivation rec {
+  pname = "nemu";
+  version = "2024-03-02";
+
+  src = ./.;
+
+  NEMU_HOME = "/build/nemu";
+  nativeBuildInputs = with pkgs; [
+    gnumake
+    flex
+    bison
+  ];
+
+  buildInputs = with pkgs; [
+    check
+    readline
+    libllvm
+  ];
+
+  configurePhase = ''
+    echo pwd=$(pwd)
+    make alldefconfig
+  '';
+
+  buildPhase = ''
+    make
+  '';
+
+  checkPhase = ''
+    make test
+  '';
+
+  installPhase = ''
+    mkdir -p $out/bin
+    make PREFIX=$out install
+  '';
+
+  meta = with lib; {
+    description = "NJU EMUlator, a full system x86/mips32/riscv32/riscv64 emulator for teaching";
+    homepage = "https://github.com/NJU-ProjectN/nemu.git";
+    license = with licenses; [ ];
+    maintainers = with maintainers; [ ];
+  };
+}