Fix iOS simulator build (pytorch#25633)

Summary:
Pull Request resolved: pytorch#25633

The iOS simulator build (x86_64) is broken right now. To fix it:

1. Fix the bug in iOS.cmake
2. Disable avx2 for mobile x86_64 build

Test Plan:
1. The `build_ios.sh` can be run successfully for iOS x86 build. The build script I'm using:

	```shell
   	./scripts/build_ios.sh \
   	-DBUILD_CAFFE2_MOBILE=OFF \
	-DIOS_PLATFORM=SIMULATOR \
   	-DUSE_NNPACK=OFF \
   	-DCMAKE_PREFIX_PATH=$(python -c 'from distutils.sysconfig import get_python_lib; print(get_python_lib())') \
   	-DPYTHON_EXECUTABLE=$(python -c 'import sys; print(sys.executable)')
   	```
2. All generated static libs are x86 libs as shown below

	```
	> lipo -i *.a
	Non-fat file: libasmjit.a is architecture: x86_64
	Non-fat file: libc10.a is architecture: x86_64
	Non-fat file: libcaffe2_protos.a is architecture: x86_64
	Non-fat file: libclog.a is architecture: x86_64
	Non-fat file: libcpuinfo.a is architecture: x86_64
	Non-fat file: libfbgemm.a is architecture: x86_64
	Non-fat file: libtorch.a is architecture: x86_64

Differential Revision: D17183803

Pulled By: xta0

fbshipit-source-id: 870d5433a3616b8e7ed9fb7dfab6aebbda26f723

cmake/MiscCheck.cmake

@@ -154,31 +154,32 @@ endif()
 cmake_pop_check_state()
 
 # ---[ Check if the compiler has AVX/AVX2 support. We only check AVX2.
-cmake_push_check_state(RESET)
-if (MSVC)
-  set(CMAKE_REQUIRED_FLAGS "/arch:AVX2")
-else()
-  set(CMAKE_REQUIRED_FLAGS "-mavx2")
-endif()
-CHECK_CXX_SOURCE_COMPILES(
-    "#include <immintrin.h>
-     int main() {
-       __m256i a, b;
-       a = _mm256_set1_epi8 (1);
-       b = a;
-       _mm256_add_epi8 (a,a);
-       __m256i x;
-       _mm256_extract_epi64(x, 0); // we rely on this in our AVX2 code
-       return 0;
-     }" CAFFE2_COMPILER_SUPPORTS_AVX2_EXTENSIONS)
-if (CAFFE2_COMPILER_SUPPORTS_AVX2_EXTENSIONS)
-  message(STATUS "Current compiler supports avx2 extension. Will build perfkernels.")
-  # Also see CMakeLists.txt under caffe2/perfkernels.
-  set(CAFFE2_PERF_WITH_AVX 1)
-  set(CAFFE2_PERF_WITH_AVX2 1)
+if (NOT INTERN_BUILD_MOBILE)
+  cmake_push_check_state(RESET)
+  if (MSVC)
+    set(CMAKE_REQUIRED_FLAGS "/arch:AVX2")
+  else()
+    set(CMAKE_REQUIRED_FLAGS "-mavx2")
+  endif()
+  CHECK_CXX_SOURCE_COMPILES(
+      "#include <immintrin.h>
+      int main() {
+        __m256i a, b;
+        a = _mm256_set1_epi8 (1);
+        b = a;
+        _mm256_add_epi8 (a,a);
+        __m256i x;
+        _mm256_extract_epi64(x, 0); // we rely on this in our AVX2 code
+        return 0;
+      }" CAFFE2_COMPILER_SUPPORTS_AVX2_EXTENSIONS)
+  if (CAFFE2_COMPILER_SUPPORTS_AVX2_EXTENSIONS)
+    message(STATUS "Current compiler supports avx2 extension. Will build perfkernels.")
+    # Also see CMakeLists.txt under caffe2/perfkernels.
+    set(CAFFE2_PERF_WITH_AVX 1)
+    set(CAFFE2_PERF_WITH_AVX2 1)
+  endif()
+  cmake_pop_check_state()
 endif()
-cmake_pop_check_state()
-
 # ---[ Check if the compiler has AVX512 support.
 cmake_push_check_state(RESET)
 if (MSVC)

cmake/iOS.cmake

@@ -66,7 +66,6 @@ if (${IOS_PLATFORM} STREQUAL "OS")
     # This causes the installers to properly locate the output libraries
     set (CMAKE_XCODE_EFFECTIVE_PLATFORMS "-iphoneos")
 elseif (${IOS_PLATFORM} STREQUAL "SIMULATOR")
-    set (SIMULATOR true)
     set (IOS_PLATFORM_LOCATION "iPhoneSimulator.platform")
     set (XCODE_IOS_PLATFORM iphonesimulator)
 
@@ -161,7 +160,7 @@ set (CMAKE_OSX_SYSROOT ${CMAKE_IOS_SDK_ROOT} CACHE PATH "Sysroot used for iOS su
 if (IOS_PLATFORM STREQUAL "OS")
     set (DEFAULT_IOS_ARCH "armv7;armv7s;arm64")
 elseif (IOS_PLATFORM STREQUAL "SIMULATOR")
-    set (DEFAULT_IOS_ARCH "i386;x86_64")
+    set (DEFAULT_IOS_ARCH "x86_64")
 elseif (IOS_PLATFORM STREQUAL "WATCHOS")
     set (DEFAULT_IOS_ARCH "armv7k")
 endif ()