RFR: 8375738: Fix -Wzero-as-null-pointer-constant warnings in MacOSX/bsd code

[Kim Barrett]

Please review this change to MacOSX and bsd code to avoid using literal zero
as a null pointer constant, instead using nullptr.

Also disable that warning when building the gtest framework with clang,
similarly to what was done in JDK-8338154 for gcc.

Testing: mach5 tier1.  Also tested with additional build changes to enable the
warning.

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Commit messages:
 - fix MacOSX -Wzero-as-null-pointer-constant warnings

Changes: https://git.openjdk.org/jdk/pull/29329/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29329&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375738
  Stats: 6 lines in 3 files changed: 1 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/29329.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29329/head:pull/29329

PR: https://git.openjdk.org/jdk/pull/29329