Make sure GCC uses the Sparc V9 instruction set
We only support Sparc V9 and better as V8 lacks an atomic CAS instruction which we need for SMP. This means that we have to pass -mcpu=v9 to GCC when compiling and assembling. Hardcoding the flag is hackish but seems to be our best bet at the moment. It can still be overridden by the user as GCC picks the best -mcpu flag regardless of the ordering.