Various Lua fixes

[umanwizard]

The main fixes are as follows:

• Document and make clear that hitting the bottom of the Lua stack is an unexpected condition (due to dummy values)
• Always return to the native unwinder from a cframe. Previously we didn't do so in the case that it was the root cframe (implicitly relying on the fact that we would then unwind into the dummy values described above on the next iteration, where we would then return to the native unwinder anyway).
• Fix the spadjust value for jit frames on aarch64
• Always unwind past the native frame when returning to the native unwinder.

[gnurizen]

should probably just delete this