[CMAKE] Use a stripped binary to build the dist and DMG for OSX

Summary:
This diff creates a copy of the bitcoin-qt application bundle on OSX,
and strip the executable inside.
This stripped version is then use to build the DMG image.

Depends on D4437.

Test Plan:
Build for OSX:

ninja osx-dmg

Check there is a src/qt/stripped directory containing a copy of the
bundle with a stripped binary inside.
Check the dist and DMG binary is stripped.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4438