user: implement mlibc as the libc, finally.

It's finally done..

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

user/include/mlibc/tests/rtld/ifunc/test.c

@@ -0,0 +1,10 @@
+#include <assert.h>
+
+int foo(void);
+
+int main(void) {
+	int res = foo();
+	assert(res == 420);
+
+	return 0;
+}