* cannon: Finish emulating rest of 64-bit instructions

This fixes the 64-bit stubs for various instructions (except lld/scd).

* review comments; fix dmult

* add todo

* test div by zero

* add a couple more dmultu tests

* remove dead code

* cannon: Fix remaining mips64 emulation bugs

* fix 64-bit Makefile build script; review comments

* fix build script