runtime: zero-initialize LR on new stacks

Currently we initialize LR on a new stack by writing nil to it. But
this is an initializing write since the newly allocated stack is not
zeroed, so this is unsafe with the hybrid barrier. Change this is a
uintptr write to avoid a bad write barrier.

Updates #17503.

Change-Id: I062ac352e35df7da4644c1f2a5aaab87049d1f60
Reviewed-on: https://go-review.googlesource.com/32093Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/proc.go

@@ -2806,7 +2806,7 @@ func newproc1(fn *funcval, argp *uint8, narg int32, nret int32, callerpc uintptr
 	spArg := sp
 	if usesLR {
 		// caller's LR
-		*(*unsafe.Pointer)(unsafe.Pointer(sp)) = nil
+		*(*uintptr)(unsafe.Pointer(sp)) = 0
 		prepGoExitFrame(sp)
 		spArg += sys.MinFrameSize
 	}