As another commenter pointed out, AMD owns the x86_64 patents. The original x86 patents have long since expired already, and the x64 patents should be expiring soon as well. The ones listed on Wikipedia have already expired[0] but it is not clear how many extensions are needed to run most modern x64 binaries and if they can be easily emulated via another extension or through microcode. (if that is even allowed).

[0] https://en.wikipedia.org/wiki/X86-64#Licensing

(From the above link, US6889312B1 actually had its patents mistakenly transferred to Philips which seemingly went unnoticed for 18 years)