user: implement mlibc as the libc, finally.

It's finally done..

Signed-off-by: kaguya <vpshinomiya@protonmail.com>

user/include/mlibc/options/ansi/musl-generic-math/ilogb.c

@@ -0,0 +1,26 @@
+#include <limits.h>
+#include "libm.h"
+
+int ilogb(double x)
+{
+	#pragma STDC FENV_ACCESS ON
+	union {double f; uint64_t i;} u = {x};
+	uint64_t i = u.i;
+	int e = i>>52 & 0x7ff;
+
+	if (!e) {
+		i <<= 12;
+		if (i == 0) {
+			FORCE_EVAL(0/0.0f);
+			return FP_ILOGB0;
+		}
+		/* subnormal x */
+		for (e = -0x3ff; i>>63 == 0; e--, i<<=1);
+		return e;
+	}
+	if (e == 0x7ff) {
+		FORCE_EVAL(0/0.0f);
+		return i<<12 ? FP_ILOGBNAN : INT_MAX;
+	}
+	return e - 0x3ff;
+}