[CMAKE] Cleanup the OSX platform file
Replace some hardcoded linker flags with CMake built-in variables.
This has no impact to the build.
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