8256320: ZGC: Update zDebug to support UseCompressedClassPointers

Reviewed-by: pliden

Author: stefank

src/hotspot/share/gc/z/zDebug.gdb

@@ -50,7 +50,12 @@ define zpo
     end
     printf "\t Page: %llu\n", ((uintptr_t)$obj & ZAddressOffsetMask) >> ZGranuleSizeShift
     x/16gx $obj
-    printf "Mark:  0x%016llx\tKlass: %s\n", (uintptr_t)$obj->_mark, (char*)$obj->_metadata->_klass->_name->_body
+    if (UseCompressedClassPointers)
+        set $klass = (Klass*)(void*)((uintptr_t)CompressedKlassPointers::_narrow_klass._base +((uintptr_t)$obj->_metadata->_compressed_klass << CompressedKlassPointers::_narrow_klass._shift))
+    else
+        set $klass = $obj->_metadata->_klass
+    end
+    printf "Mark:  0x%016llx\tKlass: %s\n", (uintptr_t)$obj->_mark, (char*)$klass->_name->_body
 end
 
 # Print heap page by page table index