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/fabs.c

@@ -0,0 +1,9 @@
+#include <math.h>
+#include <stdint.h>
+
+double fabs(double x)
+{
+	union {double f; uint64_t i;} u = {x};
+	u.i &= -1ULL/2;
+	return u.f;
+}