Restrict CTransaction size assertion to x86

Summary:
D3700 breaks Gitian builds since the class size may not match our expectations on x86.
This patch restricts the check to x86 since the class sizes on each architecture is not critical.

Also add an assert message to silence a warning that there isn't one.

Test Plan:
make check
TeamCity Gitian Build

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3730