MINOR: hathreads: add support for gcc < 4.7

MINOR: hathreads: add support for gcc < 4.7

Till now the use of __atomic_* gcc builtins required gcc >= 4.7. Since
some supported and quite common operating systems like CentOS 6 still
come with older versions (4.4) and the mapping to the older builtins
is reasonably simple, let's implement it.

This code is only used for gcc < 4.7. It has been quickly tested on a
machine using gcc 4.4.4 and provided expected results.

This patch should be backported to 1.8.