This is from Gone with the Wind. It's just the author's style choice. It does sound much better than the more ordinary 'hot Georgia sun'. The plural 'suns' has a symmetry with the plural 'veils' and 'mittens' before it. And the plural means 'the' is unnecessary. This choice just sounds nicer even though it's not standard.

Hello, I’m a native English speaker. I’ve never heard this expression before but I can tell you that the only time there is a reason to use the word “suns” is if you’re talking about planets that have multiple suns. As you know, Earth has one sun. But there are a few planets that have two or more suns. Hope this helps. ☺️

Hi Darla, I am from the southern US and have definitely heard references to the "hot Georgia sun". If the phrase was written, I am wondering if it was a typo (mistake). I have never heard sun in the plural. I hope this helps. :)

sun может быть в множественном в научной фантастике если у планеты несколько "солнц" но здесь имеется в виду больше солнечные дни или солнечный свет например солнечные свет утром, потом облака, потом днём свет, потом опять облака потом шикарный вечерний закат солнца -- по-русски мы не скажем "солнечные светы", но чтобы как-то это выразить автор выбрала через suns

Claire has explained it perfectly. It's very unlikely you'll encounter this sort of construction very often, but in certain types of writing and poetry, interesting/non-standard things can happen.