Enhance upcall mechanism (part 1)

[dbittman]

This PR, to some extent, completes a minimum of upcall support for Twizzler.

Currently, pre-upcall frames need to be restored by the kernel when the upcall returns (a syscall). In the future, user interrupt return instructions might make it possible to avoid a syscall here.

This PR:

• Cleans up and corrects the upcall frame generation and the thread upcall register setup
• Cleans up to the point of a first draft the upcall API and ABI.
• Sets up some proper minimal handling of upcalls for both runtimes, allowing at least for when a thread hits an exception, that thread gets sent an upcall that ultimately will cleanly terminate the thread. Thus thread crashing is more gracefully handled than before.

Future PR:

• Once the security contexts are more fully supported, we'll need to do another pass to finish the context switching feature here, and do permission checks.

[dbittman]

For @PandaZ3D : this touches some deep stuff in x86 -- it won't mess up with aarch64, I hope?