runtime: track running goroutine count
runtime: track running goroutine count

Used to use mcpu+msyscall but that's
problematic for packing into a single
atomic word.  The running goroutine count
(where running == Go code or syscall)
can be maintained separately, always
manipulated under lock.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/4767041

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