Summary:
On macOS, ninja only provides prebuilt Intel binaries, not arm64 ones. Since we
need to have a native arm64 ninja to be able to build binaries for the M1,
let's compile it from scratch on macOS too. Compiling it takes less than 30s so
it shouldn't add any notable cost to any builds.

Reviewed By: genevievehelsel

Differential Revision: D26322029

fbshipit-source-id: 1bbeeac2a3f0ba07c477a2b33f8878a91ccaec0e