Updated README-ppc.html with AIX-specific build instructions

[Volker Simonis]

Hi everybody,

I've just updated README-ppc.html with AIX-specific build instructions.
Everybody should now be able to build a reasonably working OpenJDK on AIX.

@colleagues at IBM:
could you please check if my write-up sounds reasonable and verify if
you can reproduce my build results. After all, you're the AIX wizards
- we're just the apprentices:)

Regards,
Volker