x/website: go.dev (index page) and go.dev/learn macOS download link downloads amd64 version

How about adding an arm64 download entry for growing M1 user base?

```
   Download packages for Windows 64-bit, MacOS (intel|apple), Linux
```  

I still see M1 users' bug reports originated from wrong tool chain installation.
I want to eliminate sources of confusion as much as possible.