[llvm] [llvm-exegesis] Add explicit support for setting DF in X86 (PR #115644)

[Clement Courbet via llvm-commits]

legrosbuffle wrote:

> EFLAGS is not in there, I'm wondering why ?

Hm wait, actually only `DF` is required to be cleared, sorry for the noise.

"The direction flag DF in the %rFLAGS register must be clear (set to “forward”
direction) on function entry and return. Other user flags have no specified role in
the standard calling sequence and are not preserved across calls"

https://github.com/llvm/llvm-project/pull/115644