PPC NCG: Use liveness information in CmmCall

We make liveness information for global registers
available on `JMP` and `BCTR`, which were the last instructions
missing. With complete liveness information we do not need to
reserve global registers in `freeReg` anymore. Moreover we
assign R9 and R10 to callee saves registers.

Cleanup by removing `Reg_Su`, which was unused, from `freeReg`
and removing unused register definitions.

The calculation of the number of floating point registers is too
conservative. Just follow X86 and specify the constants directly.

Overall on PowerPC this results in 0.3 % smaller code size in nofib
while runtime is slightly better in some tests.