runtime: output number of goroutines in GC trace
Example output:
gc26(1): 1+0+1038+0 us, 0 -> 1 MB, 19074 (88777-69703) objects, 5 gs, 184/0/0 sweeps, 0(0) handoff, 0(0) steal, 0/0/0 yields
It's useful to understand long scan times,
because goroutine traceback is slow.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rsc
https://codereview.appspot.com/136310044