Fix ILogB for subnormal values

[huoyaoyuan]

Fixes #116912

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[huoyaoyuan]

This would need to be backported to 9.0, it's a regression from 8.0. I noticed this by some user report.

[tannergooding]

This would need to be backported to 9.0, it's a regression from 8.0. I noticed this by some user report.

CC. @jeffhandley. Does this meet the bar for a backport check? .NET 10 comes out in 4ish months and .NET 9 goes end of life in 10ish months.

[jeffhandley]

/backport to release/9.0-staging

Yes, functional regressions from the previous release meet the bar.

[github-actions]

Started backporting to release/9.0-staging: https://github.com/dotnet/runtime/actions/runs/15857246813