dev-lang/ghc-7.8.1_rc1: fixed ghc uninstallation

Using ${P} is incorrect in most places, for example:
    P=ghc-7.8.1_rc1, GHC_P=ghc-7.8.20140130

Only docs get installed into directory /usr/share/${P}.

Maybe we will change it in future to always be our ${P}
and mismatch hack will go away.

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