Using the OS and Arch sum types from ghc-boot:GHC.Platform in System.Info

I am opening this ticket to resume the discussion started by @bgamari on !3607 (comment 285223) regarding the usage of a proper sum type to represent OS and Architecture identifiers in System.Info

As said by @hsyl20, ghc-boot:GHC.Platform already exports adequate data types to represent those.

The doors are open, please voice your opinion, should you have one.

To be clear, this ticket is not about the immediate deprecation of the String-based API, however unfortunate it may be.