[CMAKE] Fix -Wunused-command-line-argument when adding linker flag
[CMAKE] Fix -Wunused-command-line-argument when adding linker flag

Summary:
The previous diff D3792 made the `add_linker_flag` method able to catch
unused link flags, preventing their addition.
The following diff D3802 broke the feature while refactoring the code.
Adding the warning is not enough, it should be promoted to an error.

This diff restores the previous (correct) behavior.

Test Plan:
  mkdir buildcmake_osx && cd buildcmake_osx
  cmake -GNinja .. -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/OSX.cmake
  ninja

Check the following warning disappeared with this patch:
```
clang: warning: argument unused during compilation: '-pie'
[-Wunused-command-line-argument]
```

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3825

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