hal: split aarch64 cross fixes into own file

hosts/modules/hal/aarch64.nix

@@ -1,8 +1,13 @@
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 let
-  cfg = config.sane.hal.pine64-pinephone;
+  cfg = config.sane.hal.aarch64;
 in
 {
+  options = {
+    sane.hal.aarch64.enable = (lib.mkEnableOption "aarch64-specific hardware support") // {
+      default = pkgs.system == "aarch64-linux";
+    };
+  };
   config = lib.mkIf cfg.enable {
 
     # swap papers for zathura, since only one of these cross-compiles (TODO: enable cross compilation of papers!)

hosts/modules/hal/default.nix

@@ -1,6 +1,7 @@
 { ... }:
 {
   imports = [
+    ./aarch64.nix
     ./pine64-pinephone
     ./samsung
     ./x86_64.nix

hosts/modules/hal/pine64-pinephone/default.nix

@@ -8,7 +8,6 @@ in
   };
 
   imports = [
-    ./cross.nix
     ./kernel.nix
   ];