GHC CPU/OS support policy

Motivation

Over the years GHC accumulated some code for a number CPU architectures and OS platforms that are not continuously tested and most likely not used. Even when not actively detrimental to future development, old code introduces friction. E.g. #16882 (closed) was recently filed and is still pending with no active engagement.

Proposal

GHC would benefit from having a stance on which platforms/operating systems it intends to support. Some analysis of kernels/distributions would be required to determine the initial set of platforms that could be pruned without affecting users.

Some recent changes by @trofi in Gentoo indicate their intention to drop support for GHC on underused platforms.