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

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

Author: efriedma-quic

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