darwin: add libSystem

The new Darwin SDK pattern relies on an effectively empty, stub libc
implementation. The actual libSystem to be linked is located dynamically
based on the active SDK for the target. Independent build, host, and
target SDKs are all supported by Darwin.

The stub libSystem contains empty `include` and `lib` folders to avoid
warnings from wrappers that add those paths unconditionally, which can
turn into errors when a package is building with warnings-as-errors.

While it would be nice if a fallback libc could be provided, SDK headers
are not compatible between framework versions. Providing a fallback
risks mixing headers from different frameworks, which can result in hard
to diagnose errors involving semicolons or other punctuation.

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