[CI] Cross build for OSX

Summary:
As per title. I tested running the tests with darling but no success,
there are some syscalls unimplemented.

Test Plan: Run the CI build-osx build.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5837