Mac OS X is 64-bit only since 10.7 (Lion), which has reached End-Of-Life in October 2014.
Therefore it no longer makes sense to support exporting 32-bit binaries for Mac OS X,
and we can now default to 64-bit instead of bigger "fat" binaries.
(cherry picked from commits f04958cd5d,
42c5af5e48, 3e6f2b7d98,
and 1602e0cdb9)
87abfad7e2