Perform a dummy stwcx. when we switch contexts. The context

being switched out may hold a reservation. The stwcx. will
clear the reservation. This is architecturally recommended.

The scenario this addresses is as follows:
1. Thread 1 performs a lwarx and as such holds a reservation.
2. Thread 1 gets switched out (before doing the matching
   stwcx.) and thread 2 is switched in.
3. Thread 2 performs a stwcx. to the same reservation granule.
   This will succeed because the processor has the reservation
   even though thread 2 didn't do the lwarx.

Note that on some processors the address given the stwcx. is
not checked. On these processors the mere condition of having
a reservation would cause the stwcx. to succeed, irrespective
of whether the addresses are the same. The dummy stwcx. is
especially important for those processors.

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