locking/x86: Implement local_xchg() using CMPXCHG without the LOCK prefix

Implement local_xchg() using the CMPXCHG instruction without the LOCK prefix.
XCHG is expensive due to the implied LOCK prefix.  The processor
cannot prefetch cachelines if XCHG is used.

Signed-off-by: Uros Bizjak <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Waiman Long <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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