runtime: lift state variables from func gc to var work

runtime: lift state variables from func gc to var work

We're about to split func gc across several functions, so lift the
local variables it uses for tracking statistics and state across the
cycle into the global "work" variable.

Change-Id: Ie955f2f1758c7f5a5543ea1f3f33b222bc4b1d37
Reviewed-on: https://go-review.googlesource.com/16353
Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>