runtime: check channel's elemsize before calling race detector

When c.elemsize==0 we call raceacquire() and racerelease()
as opposed to calling racereleaseacquire()

The reason for this change is that, when elemsize==0, we don't
allocate a full buffer for the channel.  Instead of individual
buffer entries, the race detector uses the c.buf as the only
buffer entry.  This simplification prevents us following the
memory model's happens-before rules implemented in racereleaseacquire().
So, instead of calling racereleaseacquire(), we accumulate
happens-before information in the synchronization object associated
with c.buf.

The functionality in this change is implemented in a new function
called racenotify()

Fixes #42598

Change-Id: I75b92708633fdfde658dc52e06264e2171824e51
Reviewed-on: https://go-review.googlesource.com/c/go/+/271987


Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>