Fix booting of 32-bit kernels on 64-bit G5 hardware.

For rs6000, most memory insns and addi/addis do not allow GPR0 for RA
(they use literal zero there instead). So use a 'b' constraint to make
sure to have a base register other than GPR0.
GCC-4.7 and up handles this with allocating r9 instead of r0.