runtime: clean up extra M API

There are quite a few locations that get/put Ms from the extra M list,
but the API is pretty clumsy to use. Add an easier to use getExtraM /
putExtraM API.

There are only two minor semantic changes:

1. dropm no longer calls setg(nil) inside the lockextra critical
   section. It is important that this thread no longer references the G
   (and in turn M) once it is published to the extra M list and another
   thread could acquire it. But there is no reason that needs to happen
   only after lockextra.

2. extraMLength (renamed from extraMCount) is no longer protected by
   lockextra and is instead simply an atomic (though writes are still in
   the critical section). The previous readers all dropped lockextra
   before using the value they read anyway.

For #60004.

Change-Id: Ifca4d6c84d605423855d89f49af400ca07de56f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/492742
Run-TryBot: Michael Pratt <mpratt@google.com>
Commit-Queue: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

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