ZJIT: Missing single ractor patchpoint

[tekknolagi]

From John:

I found a ZJIT bug: if you make an attr_reader on a Ractor (or other shareable, but not frozen object). Other Ractors are able to access it without an exception (it should raise).
I found this mostly motivated by wanting to remove the "single ractor only" guard around getinstancevariable in the first place

[tekknolagi]

I think we can do a minimal fix first followed by figuring out shareability semantics