svm: Convert program_counter from a static variable to an argument.

svm: Convert program_counter from a static variable to an argument.

This seems to speed the machine up 20%.  Before this patch, the
compile-everything phase ("Use the new machine and compiler to
re-compile everything.") took ~67 minutes on a 3.2GHz Core i3-550.
After applying this patch, that phase took ~53 minutes (21% less).

This conversion was incomplete in the original implementation (in
6ae556b).  I backed it out in order to get a simpler machine working.