RFR(S): 8159056: [aix] Compressed class space not allocated in lower regions

[Lindenmaier, Goetz]

Hi,

On AIX, test CompressedClassPointers.java fails. This is because allocate_metaspace_compressed_klass_ptrs()
tries only a single fixed address, which fails on aix.  This change uses the loop
implemented for aarch to search for a possible location.
The change does not affect any other platform. I had to edit the aarch code, but
it's supposed to behave as before.

Please review this change. I please need a sponsor.
http://cr.openjdk.java.net/~goetz/wr16/8159056-aix_CCSTest/webrev.01/

Best regards,
  Goetz.