indications are that we can use mfence, lfence, sfence on x86_64, we will need to test and confirm this, but this should fix the build on 64 bit linux gcc at least.  need testers to confirm this.

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@3998 d0543943-73ff-0310-b7d9-9358b9ac24b2