Fix GH-16411: gmp_export() can cause overflow

We need not only to avoid the signed overflow while calculating
`bits_per_word` (reported issue), but also the unsigned overflow when
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Fix GH-16411: gmp_export() can cause overflow

We need not only to avoid the signed overflow while calculating
`bits_per_word` (reported issue), but also the unsigned overflow when
calculating `count`. While the former has a fixed threshold, the
latter does not, since it also depends on the size in base 2. Thus we
use a somewhat unconventional error message.

Closes GH-16418.

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