[libc] Don't call user comparator function for equal pointers

[abrachet]

Summary

The standard says two equal pointers must compare equal so there is no need to call the user comparator function in this case.

libc/src/stdlib/qsort.cpp

void swap(size_t i, size_t j) const {
     for (size_t b = 0; b < elem_size; ++b) {
       uint8_t temp = elem_i[b];
       elem_i[b] = elem_j[b];
       elem_j[b] = temp;
     }
   }
 
   int elem_compare(size_t i, const uint8_t *other) const {
+    // An element must compare equal to itself so we don't need to consult the
+    // user provided comparator.
+    if (get(i) == other)
+      return 0;
     return compare(get(i), other);
   }
 
   size_t size() const { return array_size; }
 
   // Make an Array starting at index |i| and size |s|.
   Array make_array(size_t i, size_t s) const {
     return Array(get(i), s, elem_size, compare);