runtime: enable StackCopyAlways

It worked at CL 134660043 on the builders,
so I believe it will stick this time.

LGTM=bradfitz
R=khr, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/141280043
syscall: fix comment in mkall_windows.bat

src/pkg -> src

LGTM=alex.brainman
R=alex.brainman
CC=golang-codereviews
https://codereview.appspot.com/139400043
runtime: on bigger maps, start iterator at a random bucket.

This change brings the iter/delete pattern down to O(n lgn) from O(n^2).

Fixes issue 8412.

before:
BenchmarkMapPop100	   50000	     32498 ns/op
BenchmarkMapPop1000	     500	   3244851 ns/op
BenchmarkMapPop10000	       5	 270276855 ns/op

after:
BenchmarkMapPop100	  100000	     16169 ns/op
BenchmarkMapPop1000	    5000	    300416 ns/op
BenchmarkMapPop10000	     300	   5990814 ns/op

LGTM=iant
R=golang-codereviews, iant, khr
CC=golang-codereviews
https://codereview.appspot.com/141270043