Fix IOS builds (pytorch#127985)

IOS builds fail these days, fix it.

Pull Request resolved: pytorch#127985
Approved by: https://github.com/ezyang

CMakeLists.txt

@@ -898,7 +898,7 @@ if(USE_SLEEF_FOR_ARM_VEC256)
 endif()
 
 # Enable sleef on macOS with Apple silicon by default
-if((${CMAKE_SYSTEM_NAME} STREQUAL "Darwin") AND (${CMAKE_SYSTEM_PROCESSOR} STREQUAL "arm64"))
+if((${CMAKE_SYSTEM_NAME} STREQUAL "Darwin") AND ("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "arm64"))
   message(STATUS "Running on macOS with Apple silicon")
   string(APPEND CMAKE_CXX_FLAGS " -DAT_BUILD_ARM_VEC256_WITH_SLEEF")
   add_definitions(-DAT_BUILD_ARM_VEC256_WITH_SLEEF)