Add an option for -fsanitize=undefined

[kimden]

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[Alayan-stk-2]

@CodingJellyfish Use "squash and merge" to merge PRs, please. Having a normal commit + a merge commit pollutes the commit history.

[qwertychouskie]

"Rebase and merge" is IMHO even better. Also I think you can set the default in the repo options.