[PowerPC] Use QEMU-compatible version of SPE accumulator save

Switch from "evaddumiaaw 0,0" to "evmwumiaa 0,0,0" when persisting the
accumulator. This has the benefit of actually being implemented in QEMU
as it is the form Linux uses for the same task.

Both instructions are functionally equivilent, as we are using them for
their side effect of copying the accumulator to GPRs rather than for the
actual math operation that they are performing.

Reviewed by: jhibbits