Fix bug #81119

The implementation for functions and operators is shared. However,
we should not be generating argument errors for operators.

Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

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

Fix newly added GMP test

Fix GMP comparison object handler

gmp_cmp() doesn't return false anymore in PHP 8 but will throw
an Error if compared to a non numeric string or another type of object.

Closes G

Fix GMP comparison object handler

gmp_cmp() doesn't return false anymore in PHP 8 but will throw
an Error if compared to a non numeric string or another type of object.

Closes GH-6553

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