Portage/GHCCore.hs: switch to ghc-7.4.1 as a default compiler

Before the patch we used ghc-6.10. It led to ancient set of depends,
like in case of quickcheck:
    https://github.com/gentoo-haskell/gentoo-haskell/pull/311

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