Previously they were disabled due to performance concerns on 32bit
arm, where 64bit atomics are often implemented via kernel traps.

Author: Roman Shaposhnik
Discussion: http://postgr.es/m/CA+ULb+uErkFuXUCCXWHYvnV5KnAyjGUzzRcPA-M0cgO+Hm4RSA@mail.gmail.com