RFR: 8337785: Fix simple -Wzero-as-null-pointer-constant warnings in x86 code

[Kim Barrett]

On Sun, 4 Aug 2024 04:36:05 GMT, Julian Waters <jwaters at openjdk.org> wrote:

> There really are a lot of these, aren't there? :P

Yeah, there were many thousand when I started, though a lot were from a small number of places in widely included
headers.  The large number of PRs is because I've been breaking things up into mostly trivial (or at least simple) changes
in focused areas, to make any given review easy.

In the home stretch.  There are a couple more like the current batch, and then some small but more complex ones,
including one or two where I think there might be an actual bug involved.

I'm only planning to enable the warning with gcc for now.  Enabling for clang would require testing on platforms I don't
have (easy, or any) access to.  I also won't be testing your windows/gcc work, so enabling the warning will likely break
that, even if all the shared and cpu-specific cases are dealt with.

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PR Comment: https://git.openjdk.org/jdk/pull/20453#issuecomment-2268120114