math: avoid runtime conversions of floating-point constants

gcc-12 with -frounding-mode will do inexact constant conversions at
runtime according to the runtime rounding mode.

in the math library we want constants to be rounding mode independent
so this patch fixes cases where new runtime conversions happen with
gcc-12.

fortunately this only affects two minor cases, the fix uses global
initializers where rounding mode does not apply.

after the patch the same amount of conversions happen with gcc-12 as
with gcc-11.

src/complex/cacosf.c

@@ -2,8 +2,10 @@
 
 // FIXME
 
+static const float float_pi_2 = M_PI_2;
+
 float complex cacosf(float complex z)
 {
 	z = casinf(z);
-	return CMPLXF((float)M_PI_2 - crealf(z), -cimagf(z));
+	return CMPLXF(float_pi_2 - crealf(z), -cimagf(z));
 }

src/complex/catanf.c

@@ -61,13 +61,15 @@ static const double DP1 = 3.140625;
 static const double DP2 = 9.67502593994140625E-4;
 static const double DP3 = 1.509957990978376432E-7;
 
+static const float float_pi = M_PI;
+
 static float _redupif(float xx)
 {
 	float x, t;
 	long i;
 
 	x = xx;
-	t = x/(float)M_PI;
+	t = x/float_pi;
 	if (t >= 0.0f)
 		t += 0.5f;
 	else