Specify that ~/.ghc/android-host/bin's ghc must be first on the path

[dominiccooney]

I wrestled with this for a couple of days trying to work out why HaskellActivity.cabal was causing incompatible options to ld (-Wl,-r emanating from the bowels of GHC, and -lopt-shared from the cabal file) and trying to make it go (add -shared to ghc-options, then link HSrts and Cffi, finally wrestling with trying to dynamically link modp from gnustl_shared.)

I ultimately realized my (perhaps obvious, in hindsight?) mistake: I was using some random ghc; putting ~/.ghc/android-host/bin first on my path and everything went swimmingly.

It would be nice if the instructions were me-proof.