Teach qemuppc to halt the cpu on _exit().

Teach qemuppc to halt the cpu on _exit().

Without this, qemu-system-ppc spins the host cpu until I close its
window.  I assume the default G3 emulation.  The emulator yields the
host cpu if I set certain flags in hid0 then msr.  The hid0 flag can
be any of DOZE, NAP, SLEEP, so I just set all 3.  I encode mfmsr and
mtmsr with .data4, because our assembler doesn't know instructions for
supervisor mode.

Also move some common symbols from .rom to .bss.  Our assembler puts
common symbols in any section.

Also clean up the file.  Delete a comment about linuxppc that is wrong
here.  Delete redundant .extern because .define is the same.