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://golang.org/cl/141280043

src/runtime/stack.c

@@ -26,7 +26,7 @@ enum
 
 	StackCache = 1,
 	
-	StackCopyAlways = 0,	// expect to be able to copy stacks 100% of the time
+	StackCopyAlways = 1,	// expect to be able to copy stacks 100% of the time
 };
 
 // Global pool of spans that have free stacks.