Fix printf format in GC config

[Luhrel]

Blocking dlang/dmd#10920

[dlang-bot]

Thanks for your pull request and interest in making D better, @Luhrel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
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Testing this PR locally

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dub run digger -- build "master + druntime#2988"

[rainers]

I don't think this change should be necessary. The check should take "default parameter promotion" into consideration, see 6.5.2.2.7: "The ellipsis notation in a function prototype declarator causes
argument type conversion to stop after the last declared parameter. The default argument
promotions are performed on trailing arguments."

The defintion of the term is in 6.5.2.2.6: "If the expression that denotes the called function has a type that does not include a prototype, the integer promotions are performed on each argument, and arguments that have type float are promoted to double. These are called the default argument promotions."