misc/ios: keep whole buffer in go_darwin_arm_exec

The existing go_darwin_arm_exec.go script does not work with Xcode 7,
not due to any significant changes, but just ordering and timing of
statements from lldb. Unfortunately the current design of
go_darwin_arm_exec.go makes it not obvious what gets stuck where, so
this moves from a moving buffer window to a complete buffer of the
lldb output.

The result is easier code to follow, and it works with Xcode 7.

Updates #12660.

Change-Id: I3b8b890b0bf4474119482e95d84e821a86d1eaed
Reviewed-on: https://go-review.googlesource.com/16634Reviewed-by: Michael Matloob <matloob@golang.org>