Many 64 bit configurations broken.

A change between r14964 and r14988 broke many of the configurations on aix-ppc64.

Problem is assertion error during bootimage build:

[java] Exception in thread "main" java.lang.ExceptionInInitializerError
[java] at java.lang.J9VMInternals.initialize(J9VMInternals.java:216)
[java] at org.jikesrvm.mm.mmtk.ActivePlan.global(ActivePlan.java:42)
[java] at org.mmtk.plan.semispace.SSCollector.global(SSCollector.java:181)
[java] at org.mmtk.plan.semispace.SSCollector.<init>(SSCollector.java:61)
[java] at org.jikesrvm.mm.mminterface.Selected$Collector.<init>(Selected.java:45)
[java] at org.jikesrvm.mm.mminterface.ProcessorContext.<init>(ProcessorContext.java:21)
[java] at org.jikesrvm.scheduler.Processor.<init>(Processor.java:291)
[java] at org.jikesrvm.scheduler.greenthreads.GreenProcessor.<init>(GreenProcessor.java:158)
[java] at org.jikesrvm.scheduler.greenthreads.GreenScheduler.initInternal(GreenScheduler.java:137)
[java] at org.jikesrvm.scheduler.Scheduler.init(Scheduler.java:309)
[java] at org.jikesrvm.VM.init(VM.java:2361)
[java] at org.jikesrvm.VM.initForTool(VM.java:109)
[java] at org.jikesrvm.VM.initForTool(VM.java:97)
[java] at org.jikesrvm.tools.header_gen.GenerateInterfaceDeclarations.main(GenerateInterfaceDeclarations.java:161)
[java] Caused by: java.lang.NegativeArraySizeException
[java] at org.mmtk.utility.GenericFreeList.resizeFreeList(GenericFreeList.java:148)
[java] at org.mmtk.utility.heap.Map.finalizeStaticSpaceMap(Map.java:229)
[java] at org.mmtk.plan.Plan.<init>(Plan.java:186)
[java] at org.mmtk.plan.Simple.<init>(Simple.java:37)
[java] at org.mmtk.plan.semispace.SS.<init>(SS.java:79)
[java] at org.jikesrvm.mm.mminterface.Selected$Plan.<clinit>(Selected.java:26)
[java] at java.lang.J9VMInternals.initializeImpl(Native Method)
[java] at java.lang.J9VMInternals.initialize(J9VMInternals.java:194)