numfmt: optimize output handling by using stdout directly

[xtqqczze]

cc: @ChrisDryden, @oech3

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/date/date-locale-hour is no longer failing!
Congrats! The gnu test tests/tail/retry is no longer failing!

[codspeed-hq]

Merging this PR will improve performance by 4.47%

⚡ 1 improved benchmark
✅ 287 untouched benchmarks
⏩ 40 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	numfmt_padding[(10000, 50)]	51.5 ms	49.3 ms	+4.47%

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_{Comparing xtqqczze:alloc-numfmt (9a82992) with main (6633ab8)}

Footnotes

1. 40 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/date/date-locale-hour is no longer failing!
Congrats! The gnu test tests/tail/retry is no longer failing!