RFR(S): 8220441: [PPC64] Clobber memory effect missing for memory barriers in atomics

[Doerr, Martin]

Hi,

the atomics in atomic_aix/linux_ppc.hpp use inline assembler memory barriers. They need a "clobber memory" effect in order to get handled correctly by C++ compilers.
We haven't seen issues so far, but we should avoid problems with future compiler versions.

Please review
http://cr.openjdk.java.net/~mdoerr/8220441_ppc_clobber_mem/webrev.00/

Best regards,
Martin