Support for XOP intrinsics

[thomcc]

https://en.wikipedia.org/wiki/XOP_instruction_set

These are present on many AMD CPUs, although moving forwards they've been abandoned, and were never supported on Intel chips.

LLVM has support for these. IMO think Rust should support them too, but if not I still think having a closed/WONTFIXed issue for it is better than no mention of it at all ever.

[Amanieu]

I'm happy to accept a PR for this if you're willing to implement it!