haskell-cabal.eclass: limit amount of parallel ghc threads down to 4

Many threads can severely degrade performance.
vikraman's stats for hoogle are:
    41 seconds with -j1
    4 minutes with -j16
    36 seconds with -j2
    33 seconds with -j8
    33 seconds with -j6
    32 seconds with -j4
  Q: is it a 8-core box?
  A: yes a core i7

Let's go conservative for now.

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