compiling for native M1

[NikolajBjorner]

Compiling z3 on an M1 does not necessarily create native binaries.
It is noted that adding

-DCMAKE_OSX_ARCHITECTURES=arm64

to the cmake configuration parameters ensures native binaries.

I wonder what is a good way to ensure native binaries by default.
For example, the cmake directory under z3 contains an architecture detection feature.

[NikolajBjorner]

I have added automatic detection for arm64 for "Darwin" to set this configuration.