[ARM64EC] Make intrin.h include arm64intrin.h.

Fixes compiling windows.h using clang's intrin.h.

clang/lib/Headers/intrin.h

diff --git a/clang/lib/Headers/intrin.h b/clang/lib/Headers/intrin.h
index e890dcd..7eb6dce 100644
--- a/clang/lib/Headers/intrin.h
+++ b/clang/lib/Headers/intrin.h
@@ -26,7 +26,7 @@
 #include <armintr.h>
 #endif
 
-#if defined(__aarch64__)
+#if defined(__aarch64__) || defined(__arm64ec__)
 #include <arm64intr.h>
 #endif