runtime: change minimum stack size to 2K.

It will be 8K on windows because it needs 4K for the OS.
Similarly, plan9 will be 4K.

On linux/amd64, reduces size of 100,000 goroutines
from ~819MB to ~245MB.

Update issue 7514

LGTM=dvyukov
R=golang-codereviews, dvyukov, khr, aram
CC=golang-codereviews
https://codereview.appspot.com/145790043
doc/go1.4.txt: reduced stack size

CC=golang-codereviews
https://codereview.appspot.com/144050043