Our gn build variables are declared in:

• //build/config/ios/ios_sdk.gni for iOS
• //build/config/ios/macos_sdk.gni for macOS

These use:

• //build/ios/ios_sdk.py for iOS
• //build/mac/find_sdk.py for macOS

We should:

1. Merge all gn rules/scripts these under a //build/apple directory.
2. Merge ios_sdk.py and find_sdk.py into a single script that does both. See Chromium's //build/config/apple/sdk_info.py for where they've done this.
3. Extract common gn rules to //build/config/apple/apple_sdk.gni and keep platform-specific rules in //build/config/apple/ios_sdk.gn and //build/config/apple/macos_sdk.gni.

Part of:
Issue: #144791