[cmake] Enable OSX cross compilation

Summary: This create a plateform file for OSX, and backports a small part of D1342 that is required for OSX compilation - the rest of the patch being about finding libraries using brew.

Test Plan:

cmake -GNinja .. -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/OSX.cmake
ninja

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1377