cmake: Set PROJECT_ARCH to the host architecture by default

cmake/cef_variables.cmake.in

@@ -26,18 +26,14 @@ endif()
 
 # Determine the project architecture.
 if(NOT DEFINED PROJECT_ARCH)
-  if(OS_WINDOWS AND "${CMAKE_GENERATOR_PLATFORM}" STREQUAL "arm64")
+  if(("${CMAKE_HOST_SYSTEM_PROCESSOR}" STREQUAL "arm64") OR
+     ("${CMAKE_CXX_COMPILER_ARCHITECTURE_ID}" STREQUAL "ARM64"))
     set(PROJECT_ARCH "arm64")
   elseif(CMAKE_SIZEOF_VOID_P MATCHES 8)
     set(PROJECT_ARCH "x86_64")
   else()
     set(PROJECT_ARCH "x86")
   endif()
-
-  if(OS_MAC)
-    # PROJECT_ARCH should be specified on Mac OS X.
-    message(WARNING "No PROJECT_ARCH value specified, using ${PROJECT_ARCH}")
-  endif()
 endif()
 
 if(${CMAKE_GENERATOR} STREQUAL "Ninja")