thread0.td_frame overwritten in cpu_startup()

[Mark Tinguely]

on startup, the initarm() sets the thread0.td_frame to a local trapframe
structure.

in arm/ARCH/ARCH_machdep.c
	thread0.td_frame = &proc0_tf;

But cpu_startup() in arm/arm/machdep.c overwrites it with a value at the end
of the kernel stack. unfortunately, that space is also used by the thread0
pcb structure.

in arm/ARCH/ARCH_machdep.c
	thread0.td_pcb = (struct pcb *)
		(thread0.td_kstack + KSTACK_PAGES * PAGE_SIZE) - 1

in arm/arm/machdep.c
	pcb->un_32.pcb32_sp = (u_int)thread0.td_kstack +
	   USPACE_SVC_STACK_TOP;

note: USPACE_SVC_STACK_TOP is defined as KSTACK_PAGES * PAGE_SIZE, so this
is the top of the kernel stack. 

	thread0.td_frame = (struct trapframe *)pcb->un_32.pcb32_sp - 1;

This td_frame assignment in arm/arm/machdep.c should be removed. Not only
did it overwrite a perfectly good trapframe, it overwrited it with memory
that is shared with the pcb.

--Mark Tinguely.