runtime: Make gc_test test extra allocated space, not total space.
runtime: Make gc_test test extra allocated space, not total space.

Testing total space fails for gccgo when not using split
stacks, because then each goroutine has a large stack, and so
the total memory usage is large.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5487068
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go