Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

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

Don't disable early binding during preloading script

We should only disable early binding during the opcache_compile_file()
calls, not inside the preloading script or anything it include

Don't disable early binding during preloading script

We should only disable early binding during the opcache_compile_file()
calls, not inside the preloading script or anything it includes.
The right condition to check for is whether we compile the file
without execution, as declaring classes is "execution".

show more ...