[CMAKE] Cleanup the OSX platform file

Summary:
Replace some hardcoded linker flags with CMake built-in variables.
This has no impact to the build.

Test Plan:

mkdir buildcmake && cd buildcmake
cmake -GNinja -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/OSX.cmake ..
ninja -v

Check the linker flags are identical before and after this patch.
Note: the build can fail if the fixes (other diffs) are not landed.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3031