[POWERPC] Avoid unpaired stwcx. on some processors

The context switch code in the kernel issues a dummy stwcx. to clear the
reservation, as recommended by the architecture.  However, some processors
can have issues if this stwcx to address A occurs while the reservation
is already held to a different address B.  To avoid this problem, the dummy
stwcx. needs to be paired with a dummy lwarx to the same address.

This adds the dummy lwarx, and creates a cpu feature bit to indicate
which cpus are affected.  Tested on mpc8641_hpcn_defconfig in
arch/powerpc; build tested in arch/ppc.

Signed-off-by: Becky Bruce <becky.bruce@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>