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.
