runtime: convert gcController.dedicatedMarkWorkersNeeded to atomic type

runtime: convert gcController.dedicatedMarkWorkersNeeded to atomic type

In gcController.startCycle we just compute the initial value in a
local variable before assigning to the atomic field to avoid noisy
churn.

For #53821.

Change-Id: Ibde0ac8fd49aa6bbee3bd02fe3ffb17429abd5a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/417784
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>