address-model=64 architecture=arm target-os=darwin generates x86_64 output

This was reported by microsoft/vcpkg#16630 and I can reproduce it on my machine with `lipo -archs libboost_system.a` which returns `x86_64`

Start by reproducing the reported configuration with B2 and verify the archive using `lipo -archs libboost_system.a`. Trace how `address-model=64`, `architecture=arm`, and `target-os=darwin` are interpreted; done means the generated archive reports the requested ARM architecture rather than `x86_64`.

Written by the indexing model from the issue text.