Fix iOS simulator build (#25633)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/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
