RFR: 8325347: Rename native_thread.h

[Kim Barrett]

Please review this trivial change that renames the file
test/hotspot/jtreg/vmTestbase/nsk/share/native/native_thread.h
to native_thread.hpp. and replaces uses of NULL in that file.

Testing: mach5 tier1

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Commit messages:
 - rename NULLs in native_thread.hpp
 - rename native_thread.h

Changes: https://git.openjdk.org/jdk/pull/17737/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17737&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325347
  Stats: 171 lines in 14 files changed: 78 ins; 78 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/17737.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17737/head:pull/17737

PR: https://git.openjdk.org/jdk/pull/17737