ia32: VCPU only needs kernelSP on ia32
ia32: VCPU only needs kernelSP on ia32

x86-64 has better ways (`swapgs`) of managing per core kernel stacks.
This commit hides the `kernelSP` member of a `vcpu_t` under x86-64
so as not to cause confusion and accidental attempted usage

cmr/seL4,cmr/seL4,cmr/seL4