xcbuild: refactor

This reworks some of xcbuild logic to make it more compatible with
Apple’s SDK.

- Add a fake version of xcrun & xcode-select
- Cleanup platform generation. Clang does not like having 20 char
  hashes in sysroot so it is much easier to just build the parent
  directory for each runCommand. This is a little awkward but I have
  renamed everything with an added ‘s’ to make the distinction more clear.
- Cleaned up wrapper.nix in some different ways
- Reuse some versioning logic so that we don’t end up with two
  different versions of Xcode or SDK reported.

pkgs/development/tools/xcbuild/sdks.nix

@@ -0,0 +1,28 @@
+{ runCommand, lib, toolchainName, sdkName, writeText, version, xcodePlatform }:
+
+let
+  inherit (lib.generators) toPlist;
+
+  SDKSettings = {
+    CanonicalName = sdkName;
+    DisplayName = sdkName;
+    Toolchains = [ toolchainName ];
+    Version = version;
+    MaximumDeploymentTarget = version;
+    isBaseSDK = "YES";
+  };
+
+  SystemVersion = {
+    ProductName = "Mac OS X";
+    ProductVersion = version;
+  };
+in
+
+runCommand "SDKs" {
+  inherit version;
+} ''
+  sdk=$out/${sdkName}.sdk
+  install -D ${writeText "SDKSettings.plist" (toPlist {} SDKSettings)} $sdk/SDKSettings.plist
+  install -D ${writeText "SystemVersion.plist" (toPlist {} SystemVersion)} $sdk/System/Library/CoreServices/SystemVersion.plist
+  ln -s $sdk $out/${xcodePlatform}.sdk
+''