modules/nixpkgs: add hostPlatform & buildPlatform options

modules/top-level/nixpkgs.nix

@@ -172,6 +172,52 @@ in
       '';
     };
 
+    hostPlatform = lib.mkOption {
+      type = with lib.types; either str attrs;
+      example = {
+        system = "aarch64-linux";
+      };
+      apply = lib.systems.elaborate;
+      defaultText = lib.literalMD ''
+        - Inherited from the "host" configuration's `pkgs`
+        - Must be specified manually when building a standalone nixvim
+      '';
+      description = ''
+        Specifies the platform where the Nixvim configuration will run.
+
+        To cross-compile, also set `nixpkgs.buildPlatform`.
+
+        Ignored when `nixpkgs.pkgs` is set.
+      '';
+    };
+
+    buildPlatform = lib.mkOption {
+      type = with lib.types; either str attrs;
+      default = cfg.hostPlatform;
+      example = {
+        system = "x86_64-linux";
+      };
+      apply =
+        value:
+        let
+          elaborated = lib.systems.elaborate value;
+        in
+        # If equivalent to `hostPlatform`, make it actually identical so that `==` can be used
+        # See https://github.com/NixOS/nixpkgs/issues/278001
+        if lib.systems.equals elaborated cfg.hostPlatform then cfg.hostPlatform else elaborated;
+      defaultText = lib.literalMD ''
+        Inherited from the "host" configuration's `pkgs`.
+        Or `config.nixpkgs.hostPlatform` when building a standalone nixvim.
+      '';
+      description = ''
+        Specifies the platform on which Nixvim should be built.
+        By default, Nixvim is built on the system where it runs, but you can change where it's built.
+        Setting this option will cause Nixvim to be cross-compiled.
+
+        Ignored when `nixpkgs.pkgs` is set.
+      '';
+    };
+
     # NOTE: This is a nixvim-specific option; there's no equivalent in nixos
     source = lib.mkOption {
       type = lib.types.path;