8215898: Build broken on 32-bit after JDK-8211425

[Stefan Johansson]

Hi all,

Please review this small fix for a build failure:

Bug: https://bugs.openjdk.java.net/browse/JDK-8215898
Webrev: http://cr.openjdk.java.net/~sjohanss/8215898/00/

Summary
The build failure is caused by always setting UseCompressedOops and 
UseCompressedClassPointers to false when AllocateOldGenAt is used. This 
needs to be guarded with LP64_ONLY.

Testing
Built the previously failing platforms and they now pass.

Thanks,
Stefan