pgsql: 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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/73193d82d7c8d849774bf6952dfb4287e213c572

Modified Files
--------------
src/backend/utils/adt/float.c                      |   59 +++++++++++++++++---
.../expected/float8-exp-three-digits-win32.out     |    4 +-
src/test/regress/expected/float8-small-is-zero.out |    4 +-
.../regress/expected/float8-small-is-zero_1.out    |    4 +-
src/test/regress/expected/float8.out               |    4 +-
5 files changed, 60 insertions(+), 15 deletions(-)