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
