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