Then who makes the coprocessor that is inserted into the die?

AMD, obviously, they're not going to let anyone mess with their lithography masks. With IP bought from ARM, to wit: It's a Cortex A5, which is a bog-standard block of IP if you need something better than a microcontroller but not really beefy either. Or you could say that TSMC makes them, just as the rest of the silicon.

(AMD also has an ARM architecture license and thus the right to design its own ARM cores but a) those were designed to be in a completely different performance class (application server) and b) they never made it to market. They're now probably tinkering on RISC-V in the background in their eternal quest to not have Intel fused to their hip by x86).