test, build: Enable -Werror=sign-compare
Summary:
test: Fix outstanding -Wsign-compare errors
refactor: Rework asmap Interpret to avoid ptrdiff_t
This is a backport of Core PR18216
It also includes a change in asmap.cpp that was missed in D9055 (PR18512)
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D9197