os/signal: fix a deadlock with syscall.AllThreadsSyscall() use
The syscall.AllThreadsSyscall() fixup mechanism needs to cooperate
with signal handling to ensure a notetsleepg() thread can wake up
to run the mDoFixup() function.
Fixes #43149
Change-Id: I6651b25bc44a4de47d3fb71d0293d51aef8b79c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/277434
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>