RFR: 8337786: Fix simple -Wzero-as-null-pointer-constant warnings in aarch64 code

[Aleksey Shipilev]

On Sat, 3 Aug 2024 23:28:33 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this trivial change that replaces some uses of literal 0 as a
> null pointer constant in aarch64 code to instead use nullptr.
> 
> Testing: mach5 tier1

Looks fine.

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Marked as reviewed by shade (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20454#pullrequestreview-2219207366