Continuing discussion in #495 and related to #399.

stack's understanding of GHC is a bit primitive as it only knows about version, OS, and architecture. It would help to augment this understanding to other [potentially arbitrary] factors. The current use-case is GHC bindists built with integer-simple instead of integer-gmp. Since this concept is orthogonal to version, OS, and architecture, stack has no good way to represent it. Thus, stack cannot currently

• represent the constraint via the yaml file or command-line arguments
• install specialized GHCs automatically
• check for existing presence

Currently the resolver is responsible for determining which version of GHC to use, and something else determines which OS/architecture distribution to use.