runtime: convert schedt.gcwaiting to atomic type

runtime: convert schedt.gcwaiting to atomic type

Note that this replaces numerous unsynchronized loads throughout the
scheduler.

For #53821.

Change-Id: Ica80b04c9e8c184bfef186e549526fc3f117c387
Reviewed-on: https://go-review.googlesource.com/c/go/+/419447
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>