Support WSL better

Maybe with a GHC compiled with --disable-large-address-space and OS detection via https://stackoverflow.com/questions/38859145/detect-ubuntu-on-windows-vs-native-ubuntu-from-bash-script

Fwiw, I've been getting a couple reports that disabling LBA is no longer necessary for WSL in current W10 builds as it doesn't provide any speedup; using the default GHC build configuration with LBA doesn't appear to exhibit any noticeable slowdowns. I've also updated the description at https://launchpad.net/~hvr/+archive/ubuntu/ghc-wsl to that effect.