Add a note to cheri.h that in the CHERI trusted stack frame, $pcc should
really include $pc.
Set up $c11, the stack capability, when configuring initial capability
registers for a new process on execve().  We simply set it up to point
to the same global state we install in $c0.  In the future we may want
to do fancier things, such as set ephemeral status on the capability.

While here, do a little cleanup on capability initialisation routines.
Teach userspace-capability invoke() to set up a temporary $idc around
CCall to save $c0, the stack pointer, the stack capability, and the
original $idc, which are then restored when CCall returns.  This means
we no longer need to conflate $c0 and $idc, and also properly preserve
the stack capability over object-capability invoke.  It also removes
a dependence on the CCall mechanism saving/restoring $sp.
Now that cheri_invoke() doesn't require $sp to be preserved by CCall/
CReturn, don't save/restore it in the kernel's exception handler.
Clear $sp before invoking an object capability: we save it already, and
the callee shouldn't receive it.