mpy-cross: Build an arm64 (M1) bin and a universal bin

[jepler]

Untested, as I don't have mac hardware. However, Linux identifies the resulting files as expected:

mpy-cross:                 Mach-O 64-bit x86_64 executable, flags:<NOUNDEFS|DYLDLINK|TWOLEVEL|PIE>
mpy-cross-arm64:           Mach-O 64-bit arm64 executable, flags:<NOUNDEFS|DYLDLINK|TWOLEVEL|PIE>
mpy-cross-macos-universal: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit x86_64 executable, flags:<NOUNDEFS|DYLDLINK|TWOLEVEL|PIE>] [arm64:Mach-O 64-bit arm64 executable, flags:<NOUNDEFS|DYLDLINK|TWOLEVEL|PIE>]

In theory, the x86_64 versions still work with catalina (and up, presumably), but the arm64 version (and maybe the universal version?) would be big sur only.

We should probably only merge this after positive testing from both x86_64 and m1 users.

[dhalbert]

I can test on M1.

[dhalbert]

Artifacts tested on M1 Mac Big Sur:
mpy-cross, mpy-cross-arm64, and mpy-cross-macos-universal

Artifacts tested on x64 Mac Big Sur:
mpy-cross, mpy-cross-macos-universal

Artifacts tested on x64 Mac High Sierra:
mpy-cross, mpy-cross-macos-universal

All worked and produced identical .mpy files.

[jepler]

Thanks for the testing!