Recognize unsafe unaligned getters/setters

[nbhuiyan]

This PR re-introduces changes in #19420 and addresses the review comments received there.

Contrary to the discussion in #19420, I have not found any evidence that unaligned access is not permitted in Z only. Existing CG flag SupportsAlignedAccessOnly has been used for some versions of Power as well as aarch64 and arm32. Supporting unaligned access is something that most platforms are capable of, and therefore the default behaviour should be to transform the unaligned access operation methods, unless a back-end sets the SupportsAlignedAccessOnly flag.