user: implement mlibc as the libc, finally.

It's finally done..

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

user/include/mlibc/tests/glibc/error_at_line.c

@@ -0,0 +1,8 @@
+#include <errno.h>
+#include <error.h>
+
+int main() {
+	error_at_line(0, EINVAL, "error_at_line", 5, "test: %s", "error");
+
+	return 0;
+}