Adjust degree-based trig functions for more portability.

Adjust degree-based trig functions for more portability.

The buildfarm isn't very happy with the results of commit e1bd684a34c11139.
To try to get the expected exact results everywhere:

* Replace M_PI / 180 subexpressions with a precomputed constant, so that
the compiler can't decide to rearrange that division with an adjacent
operation.  Hopefully this will fix failures to get exactly 0.5 from
sind(30) and cosd(60).

* Add scaling to ensure that tand(45) and cotd(45) give exactly 1; there
was nothing particularly guaranteeing that before.

* Replace minus zero by zero when tand() or cotd() would output that;
many machines did so for tand(180) and cotd(270), but not all.  We could
alternatively deem both results valid, but that doesn't seem likely to
be what users will want.