runtime: recursively disallow write barriers in sysmon

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>