user: implement mlibc as the libc, finally.

It's finally done..

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

user/include/mlibc/tests/rtld/symver/libfoo.c

@@ -0,0 +1,15 @@
+int foo_v1(void) {
+	return 1;
+}
+
+int foo_v2(void) {
+	return 2;
+}
+
+int foo_v3(void) {
+	return 3;
+}
+
+asm(".symver foo_v1, foo@FOO_1");
+asm(".symver foo_v2, foo@FOO_2");
+asm(".symver foo_v3, foo@@FOO_3"); // default version (due to @@ vs @)