cmake: detect cpu architecture in SDL_DetectCPUArchitecture

Stop looking for arch, when one is found

cmake/sdlplatform.cmake

@@ -53,3 +53,24 @@ macro(SDL_DetectCMakePlatform)
     set(${SDL_CMAKE_PLATFORM} TRUE)
   endif()
 endmacro()
+
+function(SDL_DetectCPUArchitecture)
+  set(archs x86 x64 arm32 arm64 loongarch64)
+  set(found FALSE)
+  foreach(arch ${archs})
+    string(TOUPPER "${arch}" arch_upper)
+    set(var_name "SDL_CPU_${arch_upper}")
+    if(found)
+      set(${var_name} 0 PARENT_SCOPE)
+    else()
+      check_cpu_architecture(${arch} ${var_name})
+      set(found ${${var_name}})
+      set(${var_name} ${${var_name}} PARENT_SCOPE)
+      set(SDL_CPU_NAME ${arch})
+    endif()
+  endforeach()
+  if(NOT found)
+    message(AUTHOR_WARNING "Unknown architecture (failed archs=${archs})")
+    set(SDL_CPU_NAME "(unknown)")
+  endif()
+endfunction()