Fix CTFE error in biguintcore.d

[dhawal543]

This PR fixes a compile-time function evaluation (CTFE) error in biguintcore.d.

The error occurs because GC.free() cannot be interpreted at compile time.
I've added checks using __ctfe to ensure the free() operations only happen
at runtime, while allowing the functions to still be usable in CTFE.

This allows the affected functions to be used both at compile time and
runtime without errors.

[dlang-bot]

Thanks for your pull request and interest in making D better, @dhawal543! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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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 + phobos#10658"