Followed a tip from
http://forum.osdev.org/viewtopic.php?f=1&t=20150
about moving the location of labels around, that made the linker error
go away and it looks like that fixed the restarting problem.
This commit also adds debugging flags to the build, and adds a debug
make target with the right commands to run gdb under qemu, which I'll
probably want to do in the future.