MacOS release builds

[yawn]

Attempt to fix #3818 partially.

This is currently just a 1:1 copy of the windows release without any attempt to DRY up pinned checkout version (or other dependencies).

When triggered appropriately this will build zstd using the same structure as the windows equivalents. Both work when I tested it manually on 26.2 on an M2.

Missing is code signing so currently users need to execute xattr -cr on the binaries.

Any preferences here on how to approach this, e.g.

1. Document xattr -cr?
2. Ad-hoc signing with codesign?
3. codesign backed by a real developer id?

Open issues:

• Decide on code signing strategy (see above)

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[yawn]

Let me know if I should followup with DRY-ing up the workflows and potentially Linux here as well or if this is entirely out of scope for PR's.