Prefer switching the memory stack from user to kernel *before* switching

Prefer switching the memory stack from user to kernel *before* switching
the register stack. While the ordering doesn't matter, it creates an
invariant not previously there: the memory stack pointer will always be
larger than the register stack pointer. With this invariant in place,
it's easier to add instrumentation code that detects a stack overflow
because in such a scenario the memory stack pointer and register stack
pointers have crossed each other.

Aside: basic kernel operation needs about half the stack size (~16K)
at most. We have plenty of head room on the kernel stack...