math: use double_t for temporaries to avoid stores on i386

When FLT_EVAL_METHOD!=0 (only i386 with x87 fp) the excess
precision of an expression must be removed in an assignment.
(gcc needs -fexcess-precision=standard or -std=c99 for this)

This is done by extra load/store instructions which adds code
bloat when lot of temporaries are used and it makes the result
less precise in many cases.
Using double_t and float_t avoids these issues on i386 and
it makes no difference on other archs.

For now only a few functions are modified where the excess
precision is clearly beneficial (mostly polynomial evaluations
with temporaries).

object size differences on i386, gcc-4.8:
             old   new
__cosdf.o    123    95
__cos.o      199   169
__sindf.o    131    95
__sin.o      225   203
__tandf.o    207   151
__tan.o      605   499
erff.o      1470  1416
erf.o       1703  1649
j0f.o       1779  1745
j0.o        2308  2274
j1f.o       1602  1568
j1.o        2286  2252
tgamma.o    1431  1424
math/*.o   64164 63635

1 files changed, 1 insertions, 1 deletions

diff --git a/src/math/__cosdf.c b/src/math/__cosdf.c
index a65f7f21..2124989b 100644
--- a/src/math/__cosdf.c
+++ b/src/math/__cosdf.c
@@ -25,7 +25,7 @@ C3  =  0x199342e0ee5069.0p-68; /*  0.0000243904487962774090654 */
 
 float __cosdf(double x)
 {
-	double r, w, z;
+	double_t r, w, z;
 
 	/* Try to optimize for parallel evaluation as in __tandf.c. */
 	z = x*x;