[llvm] [X86] Prefer `lock or` over mfence (PR #106555)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

Instead of a tuning flag I'm very tempted to say that we create a general avoidMFence() method that always returns true for 64-bit targets - similar to what we do for hasCLFLUSH() etc.

@phoebewang any thoughts?

https://github.com/llvm/llvm-project/pull/106555