Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

Extensions migrated in part 3:
* ftp
* gmp
* iconv
* opcache
* shmop

Fix dynamic function definition in preload script

We should use normal function renaming if the function is declared
during preloading itself, rather than afterwards.

This fixes

Fix dynamic function definition in preload script

We should use normal function renaming if the function is declared
during preloading itself, rather than afterwards.

This fixes a regression introduced by
68f80be9d1380de731930187250a7ed6b55ae196.

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