runtime: Use the clone system call on GNU/Linux.
runtime: Use the clone system call on GNU/Linux.

Without this we weren't supporting the standard Cloneflags
field of SysProcAttr.

R=iant
CC=gofrontend-dev
https://golang.org/cl/133630044

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