Use hardlinks when copying LLVM into the tree

[jkoritzinsky]

LLVM is quite large, so we should use hardlinks when copying the files into the tree

[ViktorHofer]

LGTM. As those files come from the package cache which is considered immutable, double check that the copied files don't get modified.

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[jkoritzinsky]

/ba-g wasm timeout (tracked by dotnet/dnceng#4823)