SPE registers are already exported in core dumps with the VMX note, so use
the same interface for live access.
Instead of simply guarding out in #ifndef __SPE__ the cpu_feature check, I
chose to keep the check and check against PPC_FEATURE_SPE, on the off-chance
someone decides to run a SPE kernel on a non-SPE device (which is possible,
though highly unlikely, and would be no different from running a MPC85XX
kernel in that instance).
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