[compiler-rt] [code-quality] replaced insecure function calls in _initializeAvailabilityCheck

[gbMattN]

Replacing the insecure functions with their more secure versions as recommended in issue 64611 (#64611)

[github-actions]

Thank you for submitting a Pull Request (PR) to the LLVM Project!

This PR will be automatically labeled and the relevant teams will be
notified.

If you wish to, you can add reviewers by using the "Reviewers" section on this page.

If this is not working for you, it is probably because you do not have write
permissions for the repository. In which case you can instead tag reviewers by
name in a comment by using @ followed by their GitHub username.

If you have received no comments on your PR for a week, you can request a review
by "ping"ing the PR by adding a comment “Ping”. The common courtesy "ping" rate
is once a week. Please remember that you are asking for valuable time from other developers.

If you have further questions, they may be answered by the LLVM GitHub User Guide.

You can also ask questions in a comment on this PR, on the LLVM Discord or on the forums.

[gbMattN]

Pinging @compnerd for review (I can't manually add reviewers yet)

[compnerd]

I don't think that this adds much value, but more importantly I'm concerned about any potential ABI impact of this type of change. I think that this would need sign off from someone at apple. @Bigcheese or @vsapsai mind helping identify the right person to help with the implications for backdeployment?

[vsapsai]

@dtellenbach I believe you were doing some compiler-rt work recently. Can you please help assessing the implications for backdeployment?

[dtellenbach]

Our libc implementation doesn't support sscanf_s so unfortunately this change doesn't work for us. I appreciate that you were looking into this but given our restrictions and the relatively little value it adds this can't go in.