runtime: Fix build on systems without split stack.
runtime: Fix build on systems without split stack.

From Uros Bizjak.

R=iant
CC=gofrontend-dev
https://golang.org/cl/14438073

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