Add Mac granularity x86_64 and arm64

[qstokkink]

Starting with 8.4.0 we will have x86_64 and arm64 Mac builds. We should have the website select the right one for the visiting client.

[qstokkink]

The names are tribler-8.4.0-i386.dmg and tribler-8.4.0-arm.dmg. This is still a pre-release, so we don't have to edit the website just yet.

[qstokkink]

We now have an official release, so we should actually fix this now.