Add MIPS Magnum R4000 machine

This serie of patches adds the MIPS Magnum R4000 machine.
It has been tested with the official Magnum R4000 firmware, and is able to run programs on floppy.
SCSI adapter (in esp.c) still needs some more work to be usable.
After these patches are applied, I'll send a patch to fix MIPS PICA 61 emulation, which will share most of the Magnum code.

Patches 1 and 2 are the Magnum emulation, and patches 3 to 6 fixes problems raised by the firmware. All patches are mostly independant.

Hervé