user: implement mlibc as the libc, finally.

It's finally done..

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

user/include/mlibc/tests/posix/rindex.c

@@ -0,0 +1,11 @@
+#include <strings.h>
+#include <assert.h>
+
+int main() {
+  char str[] = "This is a sample string";
+  char *pch;
+  pch = rindex(str, 's');
+  // The last occurence of 's' is at position 18
+  assert(pch - str + 1 == 18);
+  return 0;
+}