Default to defining endian-conversion DECLs in compat w/o config

Summary:
Backport of Bitcoin Core PR12998
https://github.com/bitcoin/bitcoin/pull/12998

Test Plan:

make check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3966