[libc] harden memset and other memory operations

[SchrodingerZhu]

• Provide better test framework to determine compiler behavior
• Provide options to enable variants of implementation with more strict cryptographic/security consideration.

[pogo59]

The "determine compiler behavior" point is about looking at how clang handles some libc implementation tactics, which makes me think that cross-project-tests might be a good place to put such a test. (I said this on the PR but it makes more sense to say it here.)