Added negative zero preprocessor constants

We had some discussions already when we started using negative
zero that it could be fragile on some compilers, and apparently
PGI is one of them. Maybe not the most important target, but it
makes sense to have a common constant in one place, and it can
also save some cycles in tight loops to clearly separate float
from double versions so we avoid extra precision conversions.

Change-Id: Id30a536b4f99f0310bfb2ec5185275c466cf5e07