Ensure PPCAtomicQueue has proper alignment/padding on ppc64le
PPCAtomicQueue is used by pamilrts on both BGQ and POWER8/9 systems. POWER8/9 has 128 Bytes cache line size, while BGQ has 64 Byte. There is explicit 64 Byte alignment in PPCAtomicQueue that might lead to false sharing on POWER9. I haven't delved into this code but am suspicious. We should check this before 6.9.0 since POWER9 systems are becoming important.