cmd/gomobile: only install app pkgs on darwin
cmd/gomobile: only install app pkgs on darwin

Unfortunately GLES is not a common package for a linux desktop
to have installed. To avoid bind-based users having to jump
through the hoop of installing GL, disable this performance
optimization on linux for now.

Change-Id: Ic00baef6237dca360f191ec291b6c165eda7ad6e
Reviewed-on: https://go-review.googlesource.com/13874
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>

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