for next time

[Stephen Weeks]

> Fine by me, but how?  Bear in mind that the line number is an argument to
> a function call to GC_gc, so that operand is generated during the
> translation from Machine IL.  Then it's carried around for quite some time
> before I print it out.

How about a new variant in x86.Immediate.t, or possibly x86.Immediate.const?
This variant can be treated as an unknown constant, and replaced by the line
number when outputting the assembly.