runtime: use current P's race context in timer code

We were using the race context of the P that held the timer,
but since we unlock the P's timers while executing a timer
that could lead to a race on the race context itself.

Updates #6239
Updates #27707
Fixes #35906

Change-Id: I5f9d5f52d8e28dffb88c3327301071b16ed1a913
Reviewed-on: https://go-review.googlesource.com/c/go/+/209580


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>