Remove external reference to rndtonl.

[ibara]

It was removed in dlang/phobos@b4f675f
Fix Issue 22683 - core.math.rndtonl can't be linked

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22683	normal	core.math.rndtonl can't be linked

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + druntime#3676"