When building HotSpot, enable -Wzero-as-null-pointer-constant to enforce the HotSpot Style Guide's admonishment against such usage.

This is aspirational for now, as there are currently several thousand warnings generated by that option. Many are from a small number of widely included header files, but even after that there are quite a few. Some are not trivial to fix (not simply changing a "0" to "nullptr", for example). So some significant work on HotSpot will be required before this option can be enabled.