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

abs (-0.0::Float) and abs (-0.0::Double) previously returned -0.0, when
they should return 0.0. This patch fixes this.

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

Test Plan: abs (-0.0::Double) should = 0.0 instead of (-0.0)

Reviewers: ekmett, hvr, austin, rwbarton

Reviewed By: austin, rwbarton

Subscribers: phaskell, trofi, simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D145

GHC Trac Issues: #7858