dev-lang/ghc: add 7.6.3 to the tree (no prebuilt binaries yet)

http://www.haskell.org/ghc/docs/7.6.3/html/users_guide/release-7-6-3.html
This release is a target for new haskell platform.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>