ghc-package.eclass: added 'ghc-supports-threaded-runtime' to check if ghc supports threading

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
app-text/pandoc: allow building on nonthreaded runtime (fixes ppc64 support)

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