[CMAKE] Actually build the DMG image

Summary:
Now that the dist directory is appropriately set up, the DMG image can
be generated upon.
The method vary depending on whether we are cross compiling from Linux
or running on native OSX.

Depends on D4383.

Test Plan:
On linux (with the appropriate cross compilation definitions) and osx:

ninja osx-dmg

Check the DGM is generated and can install the software.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4384