Option in stack.yaml to set absolute path for GHC

I'm in the process of modifying the [TechEmpower](https://github.com/TechEmpower/FrameworkBenchmarks) benchmarks to test changes to GHC.

Of course, I want to be really sure that I'm running with my patched version of GHC. Fortunately, the benchmarks build with stack, but if I say `system-ghc: true` that then reduces the problem to controlling the PATH. If you ever find yourself in a situation like this where you can modify the stack.yaml -- but you can't or don't want to figure out the pile of scripts culminating in a call to stack -- then it would be really great to set the absolute path for GHC directly in stack.yaml and be certain that it's running with the right version.