[m-rev.] Failure to build Mercury rotd-2025-08-20 and rotd-2025-08-21

[Zoltan Somogyi]

> This doesn't work around the problem, it still doesn't work. Now I'll wait
> for Julien to fix the actual reason, like he kindly told me he would, in
> private mail.

The attached diff fixes the problem, in that it allows the Mercury system
compile cleanly in asm_fast.gc.mmsc grade. I just started its bootcheck,
but I am committing it because definitely is a step in the right direction,
and affects only .mmsc grades.

Zoltan.
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