Cross-compile Mac arm builds from x86

[timkpaine]

We were originally blocked from cross-compiling Mac ARM builds from x86 due to our requirement on using gcc, which did not support it. We enabled ARM builds by using the native Mac ARM runners. But now that we've moved to clang, we can cross-compile from x86. We should do so as right now we do not build ARM wheels for 3.8/3.9 https://github.com/Point72/csp/issues/207