Add xenon arch settings for xbox 360

[1superchip]

Adds the settings for powerpc-xenon for xbox 360.

maybe -m and -M should be explicit?

Let me know about the names.

[simonlindholm]

Is the thing here that this arch is expected to be used with a custom objdump that sets the -M default to something that decodes instructions in a different way than broadway?

I don't have any better ideas for naming, this works for me.

[1superchip]

The current settings specify -m broadway for ppc which causes the custom powerpc-xenon-pe-objdump to not disassemble certain instructions. By removing that, it disassembles the instructions. I would like to have the switch explicit but I'm not sure the exact values.

Maybe I should add a comment that it is meant for a custom objdump?

[simonlindholm]

Maybe @encounter knows what -M https://github.com/encounter/xbox360-binutils defaults to?