When running multiple iOS builds on the same host, GOIOS_DEVICE_ID
is used to distinguish the devices. To improve support,

- Only restart the particular device when invoking iostest.bash
with the -restart flag.
- Make the exec wrapper lock file per-device.

For the iOS builder.

Change-Id: Id6f222981f25036399a43c3202a393dba89d87cb
Reviewed-on: https://go-review.googlesource.com/57970
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>