I needed this with CMake. while ABI was a little confusing. Based on Boost/CMake docs 1, I was able to get a Universal (x86_64 arm64) on macOS/Xcode by defining the following:

BOOST_CONTEXT_ABI=sysv BOOST_CONTEXT_ARCHITECTURE=combined

Internally 2 it seems to include the AAPCS for arm64 and the sysv for x86_64.