Add the flag (always disabled for now) to run the script interpreter with support for 63+sign-bit integers.
This flag is only needed for activation logic, and after 63+sign-bit integers have been enabled, we can remove the flag and pretend the chain has always had 63+sign-bit integers.
Note that parsing and disassembling 63+sign-bit integers isn't supported yet, which will be added in another diff. Therefore, script_tests.json also wasn't updated.
Partial backport of lotusd#71.
Depends on D17001.