Add iMac (Retina 5K, 27-inch, Late 2015)

[mateusz-plociennik]

Checklist

If you have any non-Apple hardware components - submit your results to the Custom Hardware table.

• [x] I performed all steps to correctly run XcodeBenchmark.
• [x] I used Xcode 12.5 or above.
• [x] I attached a screenshot with a compilation time and other fields, example.
• [x] I confirm that Time column is still sorted.
• [x] The content in cells is centered.