Support `mac64arm` generally

[mathiasbynens]

The recent JSC-related patches already added some instances of mac64arm to the codebase. Now that V8 uploaders provide the relevant binaries (using mac-arm64 instead of mac64 in their filename) we should do the following:

• add V8 mac64arm support
• document mac64arm generally in the support table
• update https://github.com/GoogleChromeLabs/jsvu/blob/main/cli.js to add mac64arm support and to detect it