[PATCH 2.26 backport] PR gold/19047: Fix bug related to addend computation for MERGE sections.

[Roland McGrath]

I've committed it to 2.26 with "PR gold/19047" added to the ChangeLog
and the git commit log.
I've also changed 19047 to be for 2.26 rather than 2.25 (I'm the
reporter, and I don't care about a 2.25 backport), and marked it
fixed.


Thanks,
Roland