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/fdim.c

@@ -0,0 +1,10 @@
+#include <math.h>
+
+double fdim(double x, double y)
+{
+	if (isnan(x))
+		return x;
+	if (isnan(y))
+		return y;
+	return x > y ? x - y : 0;
+}