cmd/link/internal/ld: omit deprecated linker argument for iOS builds
cmd/link/internal/ld: omit deprecated linker argument for iOS builds

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 <993af7099cb24ce1a88d64108ae1e8b0385f2fc0@gmail.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

cloudflare/go,golang/go,akutz/go,nawawi/go,cloudflare/go,cloudflare/go,josharian/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,akutz/go,akutz/go,akutz/go,golang/go,nawawi/go,josharian/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,akutz/go,josharian/go,akutz/go,nawawi/go,CAFxX/go,golang/go,akutz/go,josharian/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,josharian/go,golang/go,josharian/go,josharian/go,nawawi/go,golang/go,josharian/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,cloudflare/go