Add two CHERI sysarch operations: CHERI_GET_STACK and CHERI_SET_STACK,
which allow userspace code to retrieve and manipulate the trusted stack
-- if it is authorised to make system calls.  This is undertested and
it's not yet clear this API gives us what we need, but it's a first cut
that should be suitable for early experimentation.
Validate the userspace-provided trusted stack pointer when setting the
trusted stack using sysarch().