[llvm] [llvm][aarch64][x86] Implement a lightweight spectre v1 mitigation, like MSVC /Qspectre (PR #116450)

[Kristof Beyls via llvm-commits]

https://github.com/kbeyls commented:

My sentiment is very much similar to what @chandlerc has written up in an earlier comment.
If we'd nonetheless would progress to implement this in LLVM, it looks like the AArch64 codegen isn't given the protection that is intended. The `csdb` barrier is quite a different barrier than the `lfence` barrier.

https://github.com/llvm/llvm-project/pull/116450