Add separate hostcall stack and switch to it during hostcalls

[tyler]

When an execution of a Lucet instance begins, we context switch off the host stack and onto a stack that is colocated next to the instance's heap. When that execution makes a host-call, we do not switch back to the host stack, but rather remain on the guest stack.

This leaves us in the following position: The amount of guest stack remaining is a guest-defined property when the guest makes a host-call. Stack overflows while in host code are fatal to the embedding process. Not good.

So, we propose the following: Host-calls switch back to the host stack. This isn't as heavyweight as the full context-switch as defined in https://github.com/fastly/lucet/blob/master/lucet-runtime/lucet-runtime-internals/src/context/context_asm.S. We should really only need to save and restore the stack pointer to make this work.

For clarity: This doesn't prevent stack overflows in host-calls from being fatal, but it does prevent it from being reliant upon how much stack the guest has used.

[tyler]

Upon further thought, this doesn't interact well with the future pause and resume semantics. We'd end up trashing the call stack when we pause.

So, there needs to be a separate hostcall stack.