Add ARCHFLAGS to macOS installation command

Summary:
This PR solves the installation error `Unsupported architecture` on macOS.

Related Issues:
#2288

#2285

Pull Request resolved: #2556

Reviewed By: theschnitz

Differential Revision: D26122308

Pulled By: ppwwyyxx

fbshipit-source-id: 5f8b0615f965ba399f09eaefbbe2f4ff67b04d16

Author: apivovarov

INSTALL.md

@@ -20,7 +20,7 @@ git clone https://github.com/facebookresearch/detectron2.git
 python -m pip install -e detectron2
 
 # Or if you are on macOS
-CC=clang CXX=clang++ python -m pip install ......
+CC=clang CXX=clang++ ARCHFLAGS="-arch x86_64" python -m pip install ......
 ```
 
 To __rebuild__ detectron2 that's built from a local clone, use `rm -rf build/ **/*.so` to clean the