Replace NULL with nullptr in HotSpot jtreg test native code files

Umbrella issue to cover the replacement of NULL with nullptr in the native code support files for HotSpot jtreg tests.

Outside of vmTestbase there are about 1700 occurrences in about 200 files. In vmTestbase there are over 6000 occurrences in about 650 files.

Because of the large number of files and occurrences, this should be broken up into more manageable chunks by sub-tasks.

This addresses a category of NULL uses that wasn't dealt with by JDK-8299837.