Add FreeBSD build to GitHub Actions

[jchv]

CirrusCI build is failing probably because FreeBSD 12 is too old.

This PR adds a GitHub workflow using a VM to do a build with FreeBSD 14 and moves the GCC compiler used for the FreeBSD build up to 14 as well. The duration of the FreeBSD build appears to be in the same ballpark as the Ubuntu build (probably a bit longer on average, but in some cases it will be shorter. Around 4 minutes.)

Alternatives would be:

• Update CirrusCI workflow instead, but that's harder to debug. Plus, wouldn't it be easier if there was only one CI system?

• Remove the FreeBSD build altogether. But that's lame.

You can see a test workflow here: https://github.com/jchw-forks/bsnes/actions/runs/17354431653

[Screwtapello]

I feel a little bit weird depending on Some Guy's action for FreeBSD integration, but I guess Cirrus CI amounts to being Some Guy too, so we're not really any worse off (and yes, one CI system is nicer).