numfmt: reject %f values too large to format exactly

[pocopepe]

Fixes #11936

%N.Mf was silently lossy for very large ints via the f64 path. Now bails out when the value would need 20+ digits to print, like GNU does.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/pr/bounded-memory is now passing!
Congrats! The gnu test tests/unexpand/bounded-memory is now passing!