math: excess precision fix modf, modff, scalbn, scalbnf

old code was correct only if the result was stored (without the
excess precision) or musl was compiled with -ffloat-store.
now we use STRICT_ASSIGN to work around the issue.
(see note 160 in c11 section 6.8.6.4)

src/math/modff.c

@@ -1,5 +1,4 @@
-#include <math.h>
-#include <stdint.h>
+#include "libm.h"
 
 float modff(float x, float *iptr)
 {
@@ -33,5 +32,6 @@ float modff(float x, float *iptr)
 	}
 	u.n &= ~mask;
 	*iptr = u.x;
-	return x - *iptr;
+	STRICT_ASSIGN(float, x, x - *iptr);
+	return x;
 }