amd64: remove assertion about sizeof(struct pcb)

[kib]

Summary

We no longer put pcb on stack, it is part of the struct thread.
Similarly, we do not put user fpu save area on stack.  There is no
constraints on the pcb size due to the XSAVE area required alignment.

[jhb]

Speaking of this btw, it might be nice to fix other architectures to also pull the pcb out of the kstack in general. Certainly for CHERI we prefer that arrangement.

[jhb]

In D55931#1279683, @jhb wrote:

Speaking of this btw, it might be nice to fix other architectures to also pull the pcb out of the kstack in general. Certainly for CHERI we prefer that arrangement.

(Also, it's probably too hard to do now as so many places like debuggers know the type name, but struct pcb should really be renamed struct tcb... missed that in 5.0-CURRENT :( )

[kib]

In D55931#1279683, @jhb wrote:

Speaking of this btw, it might be nice to fix other architectures to also pull the pcb out of the kstack in general. Certainly for CHERI we prefer that arrangement.

The amd64 change was made quite a time ago. I do not intent to do it for i386.
Maintainers of other arches have a lot of time. The change would require some asm adjustments, I suspect, I have no realistic way to test it on non-amd64.