Some 32-bit powerpc variants (e500 in particular) do not handle
lwsync's opcode, and instead trigger an Illegal Instruction (Program) exception.
As the only signifcant powerpc architectures that implement lwsync are 64-bit,
force the 32-bit powerpc atomic barriers to all be sync.
This could possibly be less pessimized to use isync in places instead, however
this change fixes booting on e500 cores. Without it, e500 cores panic in
swi_add().
This was also filed on GitHub https://github.com/concurrencykit/ck/pull/122