Make arena use hugepage if possible

Summary:
arena doesn't use huge page by default. This change will make it happen
if possible. A new paramerter is added for Arena(). If it's set, Arena
will use huge page always. If huge page allocation fails, Arena
allocation will fallback to malloc().

Test Plan:
Change util/arena_test to support huge page allocation.
Run below tests:
1. normal regression test:
  make check
2. Check if huge page allocation works
  echo 50 > /proc/sys/vm/nr_hugepages
  make check

Reviewers: sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D28647