Fix context switching.

Previously there was a triple fault, due to a combination of reasons
(e.g. rsp and rbp being ordered in the struct and in the assembly).

Now, the locks will be held __all the way until the new context__ has
been switched to, which completely eliminates any possibility that the
"pcid fault" originates here.

While I am unsure whether this will work, this could also be an
opportunity to be able to remove CONTEXT_SWITCH_LOCK fully.