While working on bringing full arm64 (Apple Silicon) support to https://github.com/desktop/desktop, I noticed app-path had its own prebuilt binary for x86_64.

This PR adds compile- scripts to build main for both x86_64 and arm64 platforms, and replaces the previous compile script with one that invokes the other two compile- and lipos the resulting binaries into a universal binary:

$ file main
main: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64] [arm64:Mach-O 64-bit executable arm64]
main (for architecture x86_64):	Mach-O 64-bit executable x86_64
main (for architecture arm64):	Mach-O 64-bit executable arm64