MINOR: plock: use an ARMv8 instruction barrier for the pause instruction

MINOR: plock: use an ARMv8 instruction barrier for the pause instruction

As suggested by @AGSaidi in issue #958, on ARMv8 its convenient to use
an "isb" instruction in pl_cpu_relax() to improve fairness. Without it
I've met a few watchdog conditions on valid locks with 16 threads,
indicating that some threads couldn't manage to get it in 2 seconds. I
never happened again with it. In addition, the performance increased
by slightly more than 5% thanks to the reduced contention.

This should be backported as far as 2.2, possibly even 2.0.

(cherry picked from commit 1e237d037b3a45ec92d1dfa80dfd2c6bd7fc3af9)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>