misc/ios: don't wait for response to lldb run in the exec wrapper

CL 106096 changed the iOS exec wrapper to directly run the binary
without waiting for a SIGINT signal, but did so in a way that
expects a "(lldb)" response from lldb in 2 seconds. Lldb might
not out output anything until the program finishes, so change the
exec wrapper to just fire and forget the the run command and go
straight to waiting for exit, successfully or otherwise.

Change-Id: I6a2dc63f9b29fe44edb3591afb048b9a8e2e0822
Reviewed-on: https://go-review.googlesource.com/106176


Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>