Branch used for DSC 180AB Capstone

Add ARCHFLAGS to macOS installation command

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

Related Issues:
https://github.com/facebookresearch/detectron2/issues/2288

https://github.com/facebookresearch/detectron2/issues/2285

Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/2556

Reviewed By: theschnitz

Differential Revision: D26122308

Pulled By: ppwwyyxx

fbshipit-source-id: 5f8b0615f965ba399f09eaefbbe2f4ff67b04d16