scudo: Use more size classes in the malloc_free_loop benchmarks.

As a result of recent changes to the Android size classes, the malloc_free_loop
benchmark started exhausting the 8192 size class at 32768 iterations. To avoid
this problem (and to make the test more realistic), change the benchmark to
use a variety of size classes.

Differential Revision: https://reviews.llvm.org/D73918