user: implement mlibc as the libc, finally.

It's finally done..

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

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

@@ -0,0 +1,20 @@
+#include <assert.h>
+#include <strings.h>
+#include <stddef.h>
+
+int main() {
+  char str[] = "This is a sample string";
+  char *pch;
+  // The character 's' is at position 4, 7, 11 and 18
+  pch = index(str, 's');
+  assert(pch - str + 1 == 4);
+  pch = index(pch + 1, 's');
+  assert(pch - str + 1 == 7);
+  pch = index(pch + 1, 's');
+  assert(pch - str + 1 == 11);
+  pch = index(pch + 1, 's');
+  assert(pch - str + 1 == 18);
+  pch = index(pch + 1, 's');
+  assert(pch == NULL);
+  return 0;
+}