Fix leaf lineno

[dalehamel]

Ruby's backtrace code gets the line number directly from the cfp->iseq reference:

https://github.com/ruby/ruby/blob/52a17bbe6d778d56cc600f73f107c1992350f877/vm_backtrace.c#L1763-L1767

This updates the logic to do the same, rather than use the same iseq we are getting the labels from (the "CME" iseq).

In order to do this though, we need BOTH the CME iseq (for labels), and the "plain" iseq from cfp->iseq (for the line number). Most of the code here pertains to ferrying an additional address through to userspace by putting it into the 6 bytes of padding on the Frame struct, which is probably more controversial than anything else.