Rename “OS X” to the newer “macOS” convention
Since 2016, with macOS 10.12 Sierra, Mac OS X has been renamed in macOS. It would be a nice if Bitcoin's macOS build instructions follow this naming convention to avoid misunderstandings.
Backport of core PR13366
Depends on D3910.
Includes name changes to the gitian documentation which are not part of
the original PR.
Test Plan: Read it.
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D3912