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This pr adds the `extract-section` option to `llvm-objcopy` as a common option. It differs from `dump-section` as it will produce a standalone object with just one section, as opposed to just the section contents. For more context as to other options considered, see https://github.com/llvm/llvm-project/pull/153265#issuecomment-3195696003. This difference in behaviour is used for DXC compatibility with `extract-rootsignature` and `/Frs`. This pr then implements this functionality for `DXContainer` objects. This is the second step of https://github.com/llvm/llvm-project/issues/150277 to implement as a compiler action that invokes `llvm-objcopy` for functionality. This also completes the implementation of `extract-rootsignature` as described in https://github.com/llvm/llvm-project/issues/149560.