[gdb/build] Fix Wimplicit-exception-spec-mismatch in clang build

[Simon Marchi]

> Thanks for mentioning this, that saved me some time :)
>
> Reproduced, with clang 13.
>
> Fixed by patch below, WDYT?
>
> Thanks,
> - Tom

It does fix the build, but I would like if somebody who knows well about
this could tell why these two delete operators don't have a built-in
declaration, like the others.  Because it looks a bit fishy like this.

Jan, would you know, or know someone who knows?

Simon