Replace local `die` by new `System.Exit.die`

Starting with GHC 7.10, System.Exit exports the new `die`
which is essentially the same as Haddock.Util.die, so this
commit changes Haddock.Util.die to be a simple re-export
of System.Exit.die. See also

  https://ghc.haskell.org/trac/ghc/ticket/9016

for more details.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>