AMD does offer custom x86 - see the steam deck, surface laptops and Xbox and PS4 and 5. Given there aren't a ton of small fish making custom parts they are excellent at what they are made for.

AMD is pushing x86 to Apple ARM levels that keep power use low enough (best I've seen is 16 hour battery life on a device - I think MacBooks best this still) but performance per watt I haven't seen ARM really top charts. They are awesome and I want arm and risc-v to really shine in laptops but the only player on the PC side is Qualcomm who was told to destroy their only flagship by ARM.