WP Super Cache: Fix fatal error when getmypid() is unavailable in deb…

[shankangke]

Proposed changes:

Add function_exists check before calling getmypid() to prevent fatal errors in environments where the function is disabled (via disable_functions in php.ini by some hosting providers, e.g. [1] [2] [3] [4]).

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Not applicable.

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

1. Disable getmypid() in php.ini disabled_functions.
2. Go to WP Super Cache -> Debug. Click Enable Logging.
3. Before this PR: fatal error. After this PR: no error.

[github-actions]

Thank you for your PR!

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• ✅ Add testing instructions.
  
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[github-actions]

This PR has been marked as stale. This happened because:

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If this PR is still useful, please do a [trunk merge or rebase](https://github.com/Automattic/jetpack/blob/trunk/docs/git-workflow.md#keeping-your-branch-up-to-date) and otherwise make sure it's up to date and has clear testing instructions. You may also want to ping possible reviewers in case they've forgotten about it. Please close this PR if you think it's not valid anymore — if you do, please add a brief explanation.

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