RFR(XS): 8223248: [AIX] Remove old xlC 10 workaround for load acquire

[Doerr, Martin]

Hi,

the load acquire implementation in orderAccess_aix_ppc.hpp still contains a workaround from the time in which we were using xlC 10.1:
We use lwsync instead of twi+isync which is used on linux.
This compiler is no longer supported and the VM has been tested with xlClang++ 16.1 (JDK13) without the workaround.
(Already JDK8-12 require at least xlC 12.01. Also see https://wiki.openjdk.java.net/display/Build/Supported+Build+Platforms)

I'd like AIX to use the same implementation as linux. Please review:
http://cr.openjdk.java.net/~mdoerr/8223248_remove_XLC10_workaround/webrev.00/

Best regards,
Martin