use armv7 memory barriers by default

armv7 and later provide weaker cache coherency models than armv6 and
earlier, so we cannot just implement memory barriers as no-ops.  This
patch uses the DMB instruction (or the equivalent OS-provided barrier
function) to implement barriers.  This should fix concurrency issues
on newer chips such as the Apple A6 and A7.

If you still need to support ARMv6 devices, you should pass
"armv6=true" to make when building Avian.  Ideally, the VM would
detect what kind of CPU it was executing on at runtime and direct the
JIT compiler accordingly, but I don't know how to do that on ARM.
Patches are welcome, though!