internal/runtime/exithook: make safe for concurrent os.Exit
Real programs can call os.Exit concurrently from multiple goroutines.
Make internal/runtime/exithook not crash in that case.
The throw on panic also now runs in the deferred context,
so that we will see the full stack trace that led to the panic.
That should give us more visibility into the flaky failures on
bugs #55167 and #56197 as well.
Fixes #67631.
Change-Id: Iefdf71b3a3b52a793ca88d89a9c270eb50ece094
Reviewed-on: https://go-review.googlesource.com/c/go/+/588235 Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Russ Cox <rsc@golang.org>