PPC: Tune OS::GetRandomMmapAddr for 64-bit platforms.

- AIX has 64 bits of virtual addressing, but we limit address range to
  avoid losing precision if address is stored as a double.  This
  fixes the EquivalenceOfLoggingAndTraversal test.

- Big-endian Linux has 44 bits of virtual addressing.

- Little-endian Linux has 48 bits of virtual addressing.