Move `struct pcb` out of kernel stack.

[qwattash]

Currently struct pcb is allocated by stealing a chunk of the kernel stack. There are two issues in the current implementation.

1. We are not setting bounds on struct pcb
2. Setting bounds on struct pcb requires dealing with representability and padding.

By moving struct pcb to a separate allocation we avoid having to deal with representability issues and prevent overflow into pcb from a kernel stack pointer.

[bsdjhb]

Note that upstream FreeBSD already allocates the pcb separately for amd64 (it is allocated as a member of td_md). We can probably do similar changes for arm64 and riscv64 upstream and then loop those back into CheriBSD to reduce our diff.