Maybe someone knows of a complete list somewhere...
As far as how the kernel implements the uname system call, filling in the proper information for the uname structure is spread across each of the arch directories in the kernel. So filling in the processor name and machine type for an x86 processor is done within the arch/x86 kernel source tree, and filling in that information for a powerpc is done someplace within the arch/powerpc tree.
I would figure that the kernel source has the answer to your question, it just may be that finding it may be time consuming.