Update architecture=combined for universal 2 on darwin

[hjmallon]

It would be very helpful if Boost build supported arm64/x86_64 universal builds for Apple Silicon and Intel macs. This could go under the existing combined architecture.

It requires Xcode 12.2 and macOS 10.15.

See https://stackoverflow.com/questions/64553398/compile-boost-as-universal-library-intel-and-apple-silicon-architectures

[stale]

Thank you for your contributions. Main development of B2 has moved to https://github.com/bfgroup/b2
This issue has been automatically marked as "transition" to indicate the potential for needing transition to the new B2 development project.