Test if "get_magic_quotes_gpc()" exists before calling it

Summary:
Ref T13588. This function was deprecated in PHP 7.4 (see D20942) and removed in PHP8.

Test that it exists before calling it so we don't fatal in PHP8.

See https://discourse.phabricator-community.org/t/daemon-fails-on-php-8-0-2-in-utils-php-array-merge-call-w-fix/4568.

Test Plan: Used "|| true" to test the message in PHP7. No actual testing in PHP8, but a user reports a similar patch works.

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21549