loader: fix userboot's ability to detect a guest's interpreter

Some time after r338418, I believe with -Os/-Oz -ffunction-sections
-fdata-sections, the bootprog_interp variable that held our "$Interpreter:"
marker started getting strip from all loaders, with exception to userboot
since it used bootprog_interp to determine what flavor of userboot it was.

At some point, it had been brought to my attention that this was no longer
working and I had worked up some potential solutions to use the variable
that involved printing it out. My vague recollection is that this was
rejected, and I forgot to explore the alternatives; I cannot find records of
this discussion anymore.

Fast forward to present day, Andrew reported that it was non-functional and
offered (effectively) this patch (sans comment) to stop the compiler from
optimizing it out by assigning it to a volatile variable. This removes
concerns about user-facing change while retaining the interpreter marker.
Furthermore, it could certainly be uglier.

Reported and tested by: Andrew Gierth <andrew_tao173.riddles.org.uk>
MFC after: 3 days