After CL 151139 introduced a plugin test, the macOS linker for iOS
outputs:

ld: warning: -flat_namespace is deprecated on iOS

Omit the -flat_namespace flag on iOS; plugins are not supported on
iOS, and unlikely to ever be.

Change-Id: I2d08f8b984efcfd442d572b4a0f3a2c95c551b9f
Reviewed-on: https://go-review.googlesource.com/c/151300
Run-TryBot: Elias Naur <elias.naur@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>