[CMAKE] Use native strip on OSX

Summary:
This will allow for running the install/strip target when cross building
for OSX.

Test Plan:

mkdir buildcmake && cd buildcmake
mkdir install
cmake -GNinja .. DCMAKE_INSTALL_PREFIX=install \
  -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/OSX.cmake
ninja install/strip

Check the executables are stripped (compare the size vs non stripped).

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4378