pgsql: Yet further adjust degree-based trig functions for more portabil

Yet further adjust degree-based trig functions for more portability.

Buildfarm member cockatiel is still saying that cosd(60) isn't 0.5.
What seems likely is that the subexpression (1.0 - cos(x)) isn't being
rounded to double width before more arithmetic is done on it, so force
that by storing it into a variable.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/00347575e2754b1aaacd357776560803564d3f35

Modified Files
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src/backend/utils/adt/float.c |    4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)