Update comments about _start, the kernel entry point, to reflect new
parameters needed for smp support.
If we are not the boot processor, jump to the smp startup code instead.
Implement a per-cpu panic stack, which is used for bootstrapping both
primary and secondary processors and during faults on the kernel stack.
Arrange the per-cpu page like the pcb, with the struct pcpu at the end
of the page and the panic stack before it.
Use the boot processor's panic stack for calling sparc64_init.
Split the code to set preloaded global registers and to map the kernel
tsb out into functions, which non-boot processors can call.
Allocate the kstack for thread0 dynamically in pmap_bootstrap, and give
it a guard page too.