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/logb.c

@@ -0,0 +1,17 @@
+#include <math.h>
+
+/*
+special cases:
+	logb(+-0) = -inf, and raise divbyzero
+	logb(+-inf) = +inf
+	logb(nan) = nan
+*/
+
+double logb(double x)
+{
+	if (!isfinite(x))
+		return x * x;
+	if (x == 0)
+		return -1/(x*x);
+	return ilogb(x);
+}