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mstart is the entry point for new threads, so it certainly can't
interact with GC enough to have write barriers. We move the one small
piece that is allowed to have write barriers out into its own
function.

Change-Id: Id9c31d6ffac31d0051fab7db15eb428c11cadbad
Reviewed-on: https://go-review.googlesource.com/46035


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>