cmake: actually check ARM64EC

cmake/sdlcpu.cmake

@@ -1,6 +1,6 @@
 function(SDL_DetectTargetCPUArchitectures DETECTED_ARCHS)
 
-  set(known_archs EMSCRIPTEN ARM32 ARM64 LOONGARCH64 POWERPC32 POWERPC64 X86 X64)
+  set(known_archs EMSCRIPTEN ARM32 ARM64 ARM64EC LOONGARCH64 POWERPC32 POWERPC64 X86 X64)
 
   if(APPLE AND CMAKE_OSX_ARCHITECTURES)
     foreach(known_arch IN LISTS known_archs)