Create a macOS universal binary

[baronfel]

Adds a target that

• creates arch-specific macos binaries
• stitches them together using lipo to create a universal binary (thanks @rolfbjarne!)

Things we need to think about:

• how are these signed/notarized?
• how is the universal binary passed to the rest of the build process in the azdo pipelines?
• How does the azdo pipeline on mac call the build? Because we'd need it to either call this new target directly or have this target be a dependency of the existing build entry point

cc @joeloff / @marcpopMSFT

[baronfel]

Tested this on a m2 machine and it works as expected:

% file dotnet-core-uninstall 
dotnet-core-uninstall: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64] [arm64:Mach-O 64-bit executable arm64]
dotnet-core-uninstall (for architecture x86_64):	Mach-O 64-bit executable x86_64
dotnet-core-uninstall (for architecture arm64):	Mach-O 64-bit executable arm64

[joeloff]

Don't think we do notarization for this.