Build darwin-arm64 build during make dist-macos

[asg017]

The PR edits the make dist-macos script to also compile a darwin-arm64 build of litestream.

It works by applying the appropriate GOOS/GOARCH/CGO_ENABLED env vars. But the CC="gcc -target arm64-apple-macos11" env var does the real heavy lifting. The -target is an apple clang-specific flag that allows you to cross compile arm64 M1/M2 targets from a x86_64 computer. I've used it in the past to cross compile Go binaries from a MacOS Github Action runner, in this sqlite-http workflow (binaries available here).

I tested this manually on my MacOS 12.3.1 x86_64 laptop. I then scp'ed the compiled file to my Mac M1 Mini, and was able to successfully run it. Running file litestream on the darwin-arm64 output shows it's a "Mach-O 64-bit executable arm64", while the previous "normal" compiler reports "Mach-O 64-bit executable x86_64".

More than happy to move it to a separate dist-macos-arm64 Makefile target if you'd like. I re-used the gon and openssl from the amd64 target, so I think it will work?