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

[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.

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Commit messages:
 - simple s390 fixes

Changes: https://git.openjdk.org/jdk/pull/22469/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22469&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345273
  Stats: 14 lines in 6 files changed: 0 ins; 1 del; 13 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