Add an explicit trusted-stack field for total stack space in bytes, to
avoid the need to encode compile-time lengths in userspace.  Currently
the kernel uses a fixed size in ccall/creturn, but that is easy to fix
when we need it.
In the /bin/cheritest test case for trusted-stack retrieval from userspace,
validat more aspects of the stack; use the run-time acquired stack size
rather than a compile-time constant.