runtime.GC no longer triggers a STW GC. This fixes the description of
GODEBUG=gctrace=1 so it doesn't claim otherwise.

Change-Id: Ibd34a55c5ae7b5eda5c2393b9a6674bdf1d51eb3
Reviewed-on: https://go-review.googlesource.com/45131Reviewed-by: Rick Hudson <rlh@golang.org>