AIX testing needed for -Wzero-as-null-pointer-constant

[Kim Barrett]

I'd like someone with access to AIX to try out this change:
https://github.com/openjdk/jdk/pull/29497

It enables -Wzero-as-null-pointer-constant for gcc/clang.  I think all of the
shared and ppc code should be clean.  I've fixed some cases in AIX-specific
code.  There might be some I missed though, as I found those by just looking
at the code.

So I'd like someone to try building the code from that PR and report what you
find.  Once I know AIX is clean I can proceed to turning on the warning.