[cmake] Properly locate the linker when cross building for OSX

Summary: This fixes the build when FORCE_USE_SYSTEM_CLANG is set.

Test Plan:

cd depends
FORCE_USE_SYSTEM_CLANG=1 make build-osx
cd ..
mkdir buildOSX
cd buildOSX
cmake -GNinja .. -DCMAKE_TOOLCHAIN_PATH=../cmake/platforms/OSX.cmake
ninja

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15584