build: support tvOS and Mac Catalyst targets

[bvolpato]

What does this PR do?

dev build currently plans products as iOS apps and emits flat bundles. This selects deployment settings, device families, resource paths, and extension paths for iOS, tvOS, macOS, and Mac Catalyst. Mac apps use the standard Contents layout.

The Darwin SDK builder includes available Apple TV SDKs and Catalyst targets. Builds retain the SDK's actual Xcode/platform metadata and stamp the correct SDK version into Mach-O headers while preserving effective deployment minimums. Linux Catalyst can select an updated linker through XTOOL_LD64_LLD; LLD 23 is required for current SDK arm64e reexports.

This does not add an Xcode project importer, asset compiler, or App Store distribution workflow.

How was it tested?

• Ran swift test --build-system native with Swift 6.4 on Linux and make lint.
• Extracted the real Xcode 27.1 SDKs and checked target availability and recorded build metadata.
• Built a complete SwiftUI iOS Release app and both ARM64 and Intel Catalyst slices on Linux, then assembled and validated the universal Mac bundle. Checked native Mach-O platform, minimum OS, SDK version, resources, and iOS crash-symbol UUIDs.
• Compared old and new linkers against the SDK's Catalyst reexports; the updated linker succeeds without modifying SDK stubs.

Native macOS execution, tvOS asset compilation, and App Store processing remain unverified.

AI tool usage

How much of this PR was AI-assisted? (check one)

• 0 - No AI was used to write code
• 1 - I was assisted by AI. I reviewed the finished result.
• 2 - I set the AI going and left it to it; nobody has read the result - no review, or AI review only

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