The iOS exec wrapper use complicated machinery to run a iOS binary
on a device.
Running several binaries concurrently doesn't work (reliably), which
can break tests running concurrently. For my setup, the
runtime:cpu124 and sync_cpu tests can't run reliably without one of them
crashing.

Add a file lock to the exec wrapper to serialize execution.

Fixes #14318 (for me)

Change-Id: I023610e014b327f8d66f1d2fd2e54dd0e56f2be0
Reviewed-on: https://go-review.googlesource.com/21074Reviewed-by: David Crawshaw <crawshaw@golang.org>