runtime: recursively disallow write barriers in sysmon

sysmon runs without a P. This means it can't interact with the garbage
collector, so write barriers not allowed in anything that sysmon does.

Fixes #10600.

Change-Id: I9de1283900dadee4f72e2ebfc8787123e382ae88
Reviewed-on: https://go-review.googlesource.com/17006
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/proc.go

@@ -3321,6 +3321,9 @@ func checkdead() {
 // This is a variable for testing purposes. It normally doesn't change.
 var forcegcperiod int64 = 2 * 60 * 1e9
 
+// Always runs without a P, so write barriers are not allowed.
+//
+//go:nowritebarrierrec
 func sysmon() {
 	// If a heap span goes unused for 5 minutes after a garbage collection,
 	// we hand it back to the operating system.