Add AddInt63OverflowEmulated and SubInt63OverflowEmulated

Summary:
Computes 63+sign-bit addition and subtraction with overflow checks, without using built-ins. It's a fallback for when __builtin_saddll_overflow and __builtin_ssubll_overflow are not available.

Allows 63+sign-bit integers in Script down the line.

Test Plan: ninja check

Reviewers: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D9848