powerpc/booke: Don't get and use the load offset for TOC on APs

The code was a near exact copy of the code in startup, but it doesn't need
the complexity since the kernel is already relocated. With
VM_MIN_KERNEL_ADDRESS as currently set to KERNBASE, this doesn't cause a
problem, because it's a zero offset. However, when KERNBASE is changed to a
physical load address, it then has a non-zero offset, and ends up with an
invalid stack pointer, causing the AP to hang.