Ensure that J9JITGPRSpillArea is available

Declare a fictitious (and unused) field to simulate the existence of
that structure in system dumps from JVMs that predate its introduction.

Signed-off-by: Keith W. Campbell <[email protected]>

Author: keithc-ca

debugtools/DDR_VM/src/com/ibm/j9ddr/AuxFieldInfo29.dat

@@ -214,6 +214,7 @@ J9JITFrame.returnPC = required
 J9JITHashTable.buckets = required
 J9JITHashTable.end = required
 J9JITHashTable.start = required
+J9JITGPRSpillArea.unused = void*
 J9JITStackAtlas.internalPointerMap = required
 J9JITStackAtlas.localBaseOffset = required
 J9JITStackAtlas.numberOfMapBytes = required