Add the flag (always disabled for now) to run the script interpreter with support for 64-bit integers.
This flag is only needed for activation logic, and after 64-bit integers have been enabled, we can remove the flag and pretend the chain has always had 64-bit integers.
Note that parsing and disassembling 64-bit integers isn't supported yet, which will be added in another diff. Therefore, script_tests.json also wasn't updated.