RFR: 8345273: Fix -Wzero-as-null-pointer-constant warnings in s390 code [v3]

[Kim Barrett]

> Please review this change to remove -Wzero-as-null-pointer-constant warnings
> in s390 code.
> 
> Most places involve just changing literal 0 to nullptr.
> 
> Removed a dead return after ShouldNotReachHere() that is no longer needed.
> 
> Testing:
> GHA sanity test build, with and without -Wzero-as-null-pointer-constant enabled.
> I don't have the capability to run tests for this platform, so hoping someone
> from the aix-ppc maintainers can do more testing.

Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'master' into fix-s390-zero-null
 - Merge branch 'master' into fix-s390-zero-null
 - simple s390 fixes

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22469/files
  - new: https://git.openjdk.org/jdk/pull/22469/files/dc4985cd..22374e45

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22469&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22469&range=01-02

  Stats: 3117 lines in 1457 files changed: 960 ins; 887 del; 1270 mod
  Patch: https://git.openjdk.org/jdk/pull/22469.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22469/head:pull/22469

PR: https://git.openjdk.org/jdk/pull/22469