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

[Jacek Caban via cfe-commits]

https://github.com/cjacek approved this pull request.

>  (If we actually need some of these intrinsics in ARM64EC mode, we can revisit later.)

FWIW, most of them are provided by MSVC/Windows SDK via `softintrin.h` (included from `intrin.h`) as functions, not actual intrisics.

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