RFR: 8345269: Fix -Wzero-as-null-pointer-constant warnings in ppc code

[Kim Barrett]

Please review this change to remove -Wzero-as-null-pointer-constant warnings
in ppc code.

Most places involve just changing literal 0 to nullptr.

Removed a dead return after ShouldNotReachHere() that is no longer needed.

Removed some unused Address constructors that had a default address argument
with literal 0 as the default value.  These could have been changed to use
nullptr as the default value, but since they aren't used...

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.

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

Commit messages:
 - simple ppc fixes
 - remove unneeded Address ctors

Changes: https://git.openjdk.org/jdk/pull/22466/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22466&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345269
  Stats: 44 lines in 9 files changed: 0 ins; 9 del; 35 mod
  Patch: https://git.openjdk.org/jdk/pull/22466.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22466/head:pull/22466

PR: https://git.openjdk.org/jdk/pull/22466