cmd/compile: fix stack growing algorithm

The current stack growing implementation looks not right.
Specially, the line runtime/stack.go#L1068 never gets executed,
which causes many unnecessary copystack calls.

This PR is trying to correct the implementation.
As I'm not familiar with the code, the fix is just a guess.

Change-Id: I0bea1148175fad34f74f19d455c240c94d3cb78b
GitHub-Last-Rev: 57205f9
GitHub-Pull-Request: #47010
Reviewed-on: https://go-review.googlesource.com/c/go/+/332229
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>

Author: TapirLiu

src/runtime/stack.go

@@ -1064,7 +1064,9 @@ func newstack() {
 	// recheck the bounds on return.)
 	if f := findfunc(gp.sched.pc); f.valid() {
 		max := uintptr(funcMaxSPDelta(f))
-		for newsize-gp.sched.sp < max+_StackGuard {
+		needed := max + _StackGuard
+		used := gp.stack.hi - gp.sched.sp
+		for newsize-used < needed {
 			newsize *= 2
 		}
 	}