powerpc/ptrace: Give ptrace(2) access to SPE registers when available
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).