runtime: do not mark os memory as executable on windows
runtime: do not mark os memory as executable on windows

Resubmit 3c2cddfbdaec now that windows callbacks
are not generated during runtime.

Fixes issue 5494

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://codereview.appspot.com/10487043

gnanderson/go,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,apparentlymart/golang,tv42/old-go