Summary:
Make the `Float` and `Double` implementations of `signum` handle -0.0
correctly per IEEE-754.

This, together with "Make Prelude.abs handle -0.0 correctly (#7858)",
fixes Trac #7858.

Depends on D145

Signed-off-by: Alexander Berntsen <alexander@plaimi.net>

Test Plan:
signum of (-0.0) should be (-0.0) not 0.0.

Test program:

  main =
    putStrLn $ p ++ " " ++ n
    where
      f = show . signum
      p = f (-0.0 :: Double)
    n = f (0.0 :: Double)

Reviewers: ekmett, hvr, rwbarton, austin

Reviewed By: austin

Subscribers: phaskell, simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D148

GHC Trac Issues: #7858