Change CMake config to enable universal binary for Mac (#50243)

Summary:
This PR is a step towards enabling cross compilation from x86_64 to arm64.

The following has been added:
1. When cross compilation is detected, compile a local universal fatfile to use as protoc.
2. For the simple compile check in MiscCheck.cmake, make sure to compile the small snippet as a universal binary in order to run the check.

**Test plan:**

Kick off a minimal build on a mac intel machine with the macOS 11 SDK with this command:
```
CMAKE_OSX_ARCHITECTURES=arm64 USE_MKLDNN=OFF USE_QNNPACK=OFF USE_PYTORCH_QNNPACK=OFF BUILD_TEST=OFF USE_NNPACK=OFF python setup.py install
```
(If you run the above command before this change, or without macOS 11 SDK set up, it will fail.)

Then check the platform of the built binaries using this command:
```
lipo -info build/lib/libfmt.a
```
Output:
- Before this PR, running a regular build via `python setup.py install` (instead of using the flags listed above):
  ```
  Non-fat file: build/lib/libfmt.a is architecture: x86_64
  ```
- Using this PR:
  ```
  Non-fat file: build/lib/libfmt.a is architecture: arm64
  ```

Pull Request resolved: https://github.com/pytorch/pytorch/pull/50243

Reviewed By: malfet

Differential Revision: D25849955

Pulled By: janeyx99

fbshipit-source-id: e9853709a7279916f66aa4c4e054dfecced3adb1

CMakeLists.txt

@@ -505,6 +505,20 @@ if(NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE "Release" CACHE STRING "Choose the type of build from: Debug Release RelWithDebInfo MinSizeRel Coverage." FORCE)
 endif()
 
+# The below means we are cross compiling for arm64 or x86_64 on MacOSX
+if(NOT IOS AND CMAKE_SYSTEM_NAME STREQUAL "Darwin" AND CMAKE_OSX_ARCHITECTURES MATCHES "^(x86_64|arm64)$")
+  set(CROSS_COMPILING_MACOSX TRUE)
+  # We need to compile a universal protoc to not fail protobuf build
+  execute_process(COMMAND ./scripts/build_host_protoc.sh --other-flags "-DCMAKE_OSX_ARCHITECTURES=x86_64;arm64"
+                  WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
+                  RESULT_VARIABLE BUILD_HOST_PROTOC_RESULT)
+  if(NOT BUILD_HOST_PROTOC_RESULT EQUAL "0")
+    message(FATAL_ERROR "Could not compile universal protoc.")
+  endif()
+  set(PROTOBUF_PROTOC_EXECUTABLE "${PROJECT_SOURCE_DIR}/build_host_protoc/bin/protoc")
+  set(CAFFE2_CUSTOM_PROTOC_EXECUTABLE "${PROJECT_SOURCE_DIR}/build_host_protoc/bin/protoc")
+endif()
+
 # ---[ Misc checks to cope with various compiler modes
 include(cmake/MiscCheck.cmake)
 

aten/src/ATen/Version.cpp

@@ -174,6 +174,10 @@ std::string show_config() {
   ss << "  - NNPACK is enabled\n";
 #endif
 
+#ifdef CROSS_COMPILING_MACOSX
+  ss << "  - Cross compiling on MacOSX\n";
+#endif
+
   ss << "  - "<< used_cpu_capability() << "\n";
 
   if (hasCUDA()) {

cmake/MiscCheck.cmake

@@ -43,6 +43,9 @@ if(NOT INTERN_BUILD_MOBILE)
   # important because with ASAN you might need to help the compiled library find
   # some dynamic libraries.
   cmake_push_check_state(RESET)
+  if(CMAKE_SYSTEM_NAME STREQUAL "Darwin" AND CMAKE_OSX_ARCHITECTURES MATCHES "^(x86_64|arm64)$")
+    list(APPEND CMAKE_REQUIRED_FLAGS "-arch ${CMAKE_HOST_SYSTEM_PROCESSOR}")
+  endif()
   CHECK_C_SOURCE_RUNS("
   int main() { return 0; }
   " COMPILER_WORKS)

cmake/Summary.cmake

@@ -48,7 +48,9 @@ function(caffe2_print_configuration_summary)
   message(STATUS "  BUILD_TEST            : ${BUILD_TEST}")
   message(STATUS "  BUILD_JNI             : ${BUILD_JNI}")
   message(STATUS "  BUILD_MOBILE_AUTOGRAD : ${BUILD_MOBILE_AUTOGRAD}")
-
+  if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+    message(STATUS "  CROSS_COMPILING_MACOSX : ${CROSS_COMPILING_MACOSX}")
+  endif()
   message(STATUS "  INTERN_BUILD_MOBILE   : ${INTERN_BUILD_MOBILE}")
 
   message(STATUS "  USE_BLAS              : ${USE_BLAS}")