Add __sync_bool_compare_and_swap_4 for targets which don't have it.
Add __sync_bool_compare_and_swap_4 for targets which don't have it.

R=iant
CC=gofrontend-dev
https://golang.org/cl/4125044

anlhord/gofrontend,qskycolor/gofrontend,golang/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,golang/gofrontend,golang/gofrontend,anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,qskycolor/gofrontend