add significand[fl] math functions

src/math/significand.c

@@ -0,0 +1,7 @@
+#define _GNU_SOURCE
+#include <math.h>
+
+double significand(double x)
+{
+	return scalbn(x, -ilogb(x));
+}

src/math/significandf.c

@@ -0,0 +1,7 @@
+#define _GNU_SOURCE
+#include <math.h>
+
+float significandf(float x)
+{
+	return scalbnf(x, -ilogbf(x));
+}

src/math/significandl.c

@@ -0,0 +1,7 @@
+#define _GNU_SOURCE
+#include <math.h>
+
+long double significandl(long double x)
+{
+	return scalbnl(x, -ilogbl(x));
+}