RFR: 8338154: Fix -Wzero-as-null-pointer-constant warnings in gtest framework

[Kim Barrett]

Please review this change to remove -Wzero-as-null-pointer-constant warnings
produced by the gtest framework when such warnings are enabled.

Warnings from the 3rd party GoogleTest are suppressed by disabling that
warning when compiling those files.

Testing: mach5 tier1

-------------

Commit messages:
 - gtest warnings

Changes: https://git.openjdk.org/jdk/pull/20536/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20536&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338154
  Stats: 3 lines in 2 files changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20536.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20536/head:pull/20536

PR: https://git.openjdk.org/jdk/pull/20536