Let user be able to change MKLDNN "-m" flags back and forth in subseq…

…uent builds (pytorch#23608)

Summary:
Currently once user has set `USE_NATIVE_ARCH` to OFF, they will never be able to turn it on for MKLDNN again by simply changing `USE_NATIVE_ARCH`. This commit fixes this issue.

Following up 09ba4df
Pull Request resolved: pytorch#23608

Differential Revision: D16599600

Pulled By: ezyang

fbshipit-source-id: 88bbec1b1504b5deba63e56f78632937d003a1f6

cmake/Modules/FindMKLDNN.cmake

@@ -122,7 +122,9 @@ ENDIF()
 SET(WITH_TEST FALSE CACHE BOOL "" FORCE)
 SET(WITH_EXAMPLE FALSE CACHE BOOL "" FORCE)
 SET(MKLDNN_LIBRARY_TYPE STATIC CACHE STRING "" FORCE)
-IF(NOT MKLDNN_USE_NATIVE_ARCH)  # Disable HostOpts in MKL-DNN.
+IF(MKLDNN_USE_NATIVE_ARCH)  # Disable HostOpts in MKLDNN unless MKLDNN_USE_NATIVE_ARCH is set.
+  SET(ARCH_OPT_FLAGS "HostOpts" CACHE STRING "" FORCE)
+ELSE()
   IF(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
     SET(ARCH_OPT_FLAGS "-msse4" CACHE STRING "" FORCE)
   ELSE()