runtime: refactor work stealing to dedicated function

findrunnable has grown very large and hard to follow over the years.
Parts we can split out into logical chunks should help make it more
understandable and easier to change in the future.

The work stealing loop is one such big chunk that is fairly trivial to
split out into its own function, and even has the advantage of
simplifying control flow by removing a goto around work stealing.

This CL should have no functional changes.

For #43997.
For #44313.

Change-Id: Ie69670c7bc60bd6c114e860184918717829adb22
Reviewed-on: https://go-review.googlesource.com/c/go/+/307913


Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Chris Hines <chris.cs.guy@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>