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/atanf.c b/src/math/atanf.c
index 4b59509a..ac8bfd06 100644
--- a/src/math/atanf.c
+++ b/src/math/atanf.c
@@ -40,7 +40,7 @@ static const float aT[] = {
 
 float atanf(float x)
 {
-	float w,s1,s2,z;
+	float_t w,s1,s2,z;
 	uint32_t ix,sign;
 	int id;