ghc: update HEAD version

Closes #2428.

Note this does not use the most recent snapshot as that current doesn't build on 32-bit platforms due to upstream commit few hours ago.

Using 7.8.2 to bootstrap allows us to compile HEAD fine, without the compile-time error manifested in #2428. I use the already-packaged 7.8.2 here as I had difficulties trying to install the 7.8.2 binary to follow the pattern of older compilers.