runtime: don't reserve space for stack barriers if they're off

Change-Id: I79ebccdaefc434c47b77bd545cc3c50723c18b61
Reviewed-on: https://go-review.googlesource.com/31135
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mstkbar.go

@@ -148,6 +148,10 @@ var firstStackBarrierOffset = 1024
 // gcMaxStackBarriers returns the maximum number of stack barriers
 // that can be installed in a stack of stackSize bytes.
 func gcMaxStackBarriers(stackSize int) (n int) {
+	if debug.gcstackbarrieroff > 0 {
+		return 0
+	}
+
 	if firstStackBarrierOffset == 0 {
 		// Special debugging case for inserting stack barriers
 		// at every frame. Steal half of the stack for the