Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

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

Throw ValueError instead of TypeError for malformed GMP number

If the passed argument has correct type (string) but does not
have a well-formed value, throw ValueError instead of TypeErr

Throw ValueError instead of TypeError for malformed GMP number

If the passed argument has correct type (string) but does not
have a well-formed value, throw ValueError instead of TypeError.

Closes GH-6572.

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Fix #80560: Strings containing only a base prefix return 0 object

Closes GH-6549.