RFR: 8340620: Fix -Wzero-as-null-pointer-constant warnings for CompressedOops

[Kim Barrett]

Please review this change that fixes -Wzero-as-null-pointer-constant warnings
in CompressedOops code.  These all relate to CompressedOops::base().

I also added a couple of asserts to verify our assumptions about null pointer
constants being representationally zero. That isn't a Standard-conforming
assumption, but holds for all platforms we currently support. I considered,
and even explored, a couple of different options.

(1) Continue to have CompressedOops::base() be a pointer, but avoid that
assumption, being more careful about how zero-valued pointers are treated. But
that adds significant complexity that we can't test, since we don't support
any platforms needing that extra work.

(2) Change CompressedOops::base() to an integral adjustment.  This is probably
the correct approach, but is much more intrusive and wide ranging in the
changes required.  Maybe something for the future.

Testing: mach5 tier1-5
GHA testing, verifying builds on some platforms not supported by Oracle.

There are some simple changes to s390 and ppc code that I haven't tested,
beyond verifying compilation.

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

Commit messages:
 - fix setting base to null
 - fix base comparisons to 0 in shared
 - fix base comparisons to 0 in s390.ad
 - fix base comparisons to 0 in ppc.ad

Changes: https://git.openjdk.org/jdk/pull/21172/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21172&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340620
  Stats: 15 lines in 4 files changed: 6 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/21172.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21172/head:pull/21172

PR: https://git.openjdk.org/jdk/pull/21172