Bump trusted-stack size from 4 frames to 8 frames, which fits now that the
kernel uses one or both of multiple fixed TLB entries, or large TLB entries,
for the kernel stack.

(If we start experiencing occasional kernel panics due to stack overflow,
we may need to back off this a bit -- or move to a more general solution.)

This changes the ABI on the kernel's trusted-stack data structure exposed
to userspace via sysarch().  Programs using this feature (e.g., cheritest)
will need to be recompiled.  We have a further TODO to improve the ABI to
avoid this happening again in the future if we need to grow the trusted
stack further.