systems/doubles.nix: add Apple doubles

These are used in cross-compilation to iOS devices and simulators.

Fallout from #60349.

lib/systems/doubles.nix

@@ -13,9 +13,11 @@ let
 
     "i686-cygwin" "i686-freebsd" "i686-linux" "i686-netbsd" "i686-openbsd"
 
-    "x86_64-cygwin" "x86_64-darwin" "x86_64-freebsd" "x86_64-linux"
+    "x86_64-cygwin" "x86_64-freebsd" "x86_64-linux"
     "x86_64-netbsd" "x86_64-openbsd" "x86_64-solaris"
 
+    "x86_64-darwin" "aarch64-ios" "armv7a-ios" "x86_64-ios" "i686-ios"
+
     "x86_64-windows" "i686-windows"
 
     "wasm64-wasi" "wasm32-wasi"