Fix #19740: Incorrect result of BigInt * BigInt

[shove70]

https://issues.dlang.org/show_bug.cgi?id=19740

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19740	regression	Incorrect result of BigInt * BigInt

⚠️⚠️⚠️ Warnings ⚠️⚠️⚠️

• Regression or critical bug fixes should always target the stable branch. Learn more about rebasing to stable or the D release process.

To target stable perform these two steps:

1. Rebase your branch to upstream/stable:

git rebase --onto upstream/stable upstream/master

2. Change the base branch of your PR to stable

Testing this PR locally

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

dub fetch digger
dub run digger -- build "master + phobos#6965"

[thewilsonator]

Please rebase and add tests.