RFR: JDK-8306538: Zero variant build failure after JDK-8257967

[David Holmes]

On Thu, 20 Apr 2023 10:10:57 GMT, Zixian Cai <zcai at openjdk.org> wrote:

> This follows the same style of fix applied in #13512. I noticed this issue when cross-compiling zero slowdebug for riscv64. I confirmed locally that this PR fixes the linking errors.

I'll approve this to get a quick fix in place but:

1. I can't see why this should be specific to zero.
2. I think the better fix is that  os::find_builtin_agent should be guarded by `#if INCLUDE_JVMTI`. Or even moved to a JVMTI file instead.

Thanks.

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Marked as reviewed by dholmes (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13557#pullrequestreview-1393859986