runtime: move m.thread to mOS

This field is only used on Windows.

Change-Id: I12d4df09261f8e7ad54c2abd7beda669af28c8e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/207778


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>