intrin.h checks for x86_64. But the "x86_64" define is also defined for
ARM64EC, and we don't support all the intrinsics in ARM64EC mode. Fix
the preprocessor checks to handle this correctly. (If we actually need
some of these intrinsics in ARM64EC mode, we can revisit later.)

Not exactly sure how I didn't run into this issue before now... I think
I've built code that requires these headers, but maybe not since the
define fix landed.