runtime: allow write barriers in startpanic_m

We're about to start tracking nowritebarrierrec through systemstack
calls, which will reveal write barriers in startpanic_m prohibited by
various callers.

We actually can allow write barriers here because the write barrier is
a no-op when we're panicking. Let the compiler know.

Updates #22384.
For #22460.

Change-Id: Ifb3a38d3dd9a4125c278c3680f8648f987a5b0b8
Reviewed-on: https://go-review.googlesource.com/72770
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mbarrier.go

@@ -182,6 +182,8 @@ func gcmarkwb_m(slot *uintptr, ptr uintptr) {
 func writebarrierptr_prewrite1(dst *uintptr, src uintptr) {
 	mp := acquirem()
 	if mp.inwb || mp.dying > 0 {
+		// We explicitly allow write barriers in startpanic_m,
+		// since we're going down anyway. Ignore them here.
 		releasem(mp)
 		return
 	}

src/runtime/panic.go

@@ -654,6 +654,12 @@ func recovery(gp *g) {
 	gogo(&gp.sched)
 }
 
+// startpanic_m implements unrecoverable panic.
+//
+// It can have write barriers because the write barrier explicitly
+// ignores writes once dying > 0.
+//
+//go:yeswritebarrierrec
 func startpanic_m() {
 	_g_ := getg()
 	if mheap_.cachealloc.size == 0 { // very early