Add GHC 8.6.4

Added downloads available from https://downloads.haskell.org/~ghc/8.6.4/
as at the time of commit.

Have not made this recommended yet until common tools and libs are all
confirmed to work fine.

.available-versions

@@ -15,7 +15,8 @@ ghc              8.4.3
 ghc              8.4.4
 ghc              8.6.1    bad
 ghc              8.6.2
-ghc              8.6.3    latest,recommended
+ghc              8.6.3    recommended
+ghc              8.6.4    latest
 cabal-install    2.2.0.0
 cabal-install    2.4.0.0
 cabal-install    2.4.1.0  latest,recommended