[clang] [ARM64EC] Fix compilation of intrin.h in ARM64EC mode. (PR #87717)

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Moon6688 wrote:

> 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.



https://github.com/llvm/llvm-project/pull/87717