JDK-8318834 pointed out that on linux s390x, debug builds were missing debug helpers (coding with useful helper-functions from hotspot debug.cpp).
This was because we enabled some time ago linktime gc (-Wl,--gc-sections in the linker flags, plus -ffunction-sections -fdata-sections in compile flags) for linux x390x by default, which caused elimination of this coding at linktime.
But potentially linktime gc or similar size-optimizing options could be enabled on other platforms too, and not only debug builds could loose the debug helpers (however in non-debug builds they might be not that important).

On Windows we already export the debug helper symbols, so it might be good to do this on other platforms, especially Linux, too.