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]

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