libm: fma: correct zero sign with small inputs

This is a fixed version of 888796ade284.

PR: 277783
Reported by: Victor Stinner
Reviewed by: emaste
MFC after: 1 week

(cherry picked from commit 888796ade2842486d3167067e8034254c38aadd3)
(cherry picked from commit e77ad954bb825983b4346b9cc646c9c910b1be24)
(cherry picked from commit 34f746cc7f8a8dd261027a8b392b76e70adc8438)