Fix issue 21055 - core.stdc.stdarg is not nogc

[dkorpel]

This is targeting master since there is a chance certain template instantiations break when forced @nogc. I hope the test suite covers stdarg well.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21055	normal	core.stdc.stdarg is not @nogc

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 "master + druntime#3163"

[dkorpel]

It's green now