runtime: convert workType.cycles to internal atomic types

runtime: convert workType.cycles to internal atomic types

Note that this changes a non-atomic operation to atomic operation in gcStart().

For #53821

Change-Id: I754d254f6f190855144ff62151b6bae673b47867
Reviewed-on: https://go-review.googlesource.com/c/go/+/425776
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>