Typify the sdk used in toolchain selection

Summary:
Similar to the previous diff, typify the SDK.

Theres a bit of SDK and platform being used interchangeably here, especially as strings. We should perhaps follow up and typify Platform and be explicit in certain intentions here.

Easier once we've dropped buck1 support and can use these types in structs downstream.

Reviewed By: milend

Differential Revision: D51481594

fbshipit-source-id: f5ad43bf19359905e87177b7a5698a6824101d63

prelude/platforms/apple/sdk.bzl

@@ -0,0 +1,10 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under both the MIT license found in the
+# LICENSE-MIT file in the root directory of this source tree and the Apache
+# License, Version 2.0 found in the LICENSE-APACHE file in the root directory
+# of this source tree.
+
+AppleSdks = ["iphoneos", "iphonesimulator", "maccatalyst", "macosx", "visionos", "visionsimulator", "watchos", "watchsimulator"]
+
+AppleSdk = enum(*AppleSdks)