suspend client threads while the process is native

Split from #2335. See discussion there for background which I will summarize here:

Since we remove our signal handlers when we send the process native (for dr_app_stop() or -go_native_at_bb_count), it is not safe for client threads to continue executing, as they often have itimers or might call dr_safe_read() or whatnot. The proposal is to have send_all_other_threads_native() suspend them for the duration of the process being native.