cmake: simd detection clean-up for Apple multi-arch configs.

(cherry picked from commit 83bb0f9105)

CMakeLists.txt

@@ -79,6 +79,12 @@ include("${SDL3_SOURCE_DIR}/cmake/PreseedEmscriptenCache.cmake")
 
 SDL_DetectCompiler()
 SDL_DetectTargetCPUArchitectures(SDL_CPUS)
+if(APPLE AND CMAKE_OSX_ARCHITECTURES)
+  list(LENGTH CMAKE_OSX_ARCHITECTURES _num_arches)
+  if(_num_arches GREATER 1)
+    set(APPLE_MULTIARCH TRUE)
+  endif()
+endif()
 
 # Increment this if there is an incompatible change - but if that happens,
 # we should rename the library from SDL3 to SDL4, at which point this would