Running test/garbage/parser.out.

On a 4-core Lenovo X201s (Linux):
31.12u 0.60s 31.74r 	 1 cpu, no atomics
32.27u 0.58s 32.86r 	 1 cpu, atomic instructions
33.04u 0.83s 27.47r 	 2 cpu

On a 16-core Xeon (Linux):
33.08u 0.65s 33.80r 	 1 cpu, no atomics
34.87u 1.12s 29.60r 	 2 cpu
36.00u 1.87s 28.43r 	 3 cpu
36.46u 2.34s 27.10r 	 4 cpu
38.28u 3.85s 26.92r 	 5 cpu
37.72u 5.25s 26.73r	 6 cpu
39.63u 7.11s 26.95r	 7 cpu
39.67u 8.10s 26.68r	 8 cpu

On a 2-core MacBook Pro Core 2 Duo 2.26 (circa 2009, MacBookPro5,5):
39.43u 1.45s 41.27r 	 1 cpu, no atomics
43.98u 2.95s 38.69r 	 2 cpu

On a 2-core Mac Mini Core 2 Duo 1.83 (circa 2008; Macmini2,1):
48.81u 2.12s 51.76r 	 1 cpu, no atomics
57.15u 4.72s 51.54r 	 2 cpu

The handoff algorithm is really only good for two cores.
Beyond that we will need to so something more sophisticated,
like have each core hand off to the next one, around a circle.
Even so, the code is a good checkpoint; for now we'll limit the
number of gc procs to at most 2.

R=dvyukov
CC=golang-dev
https://golang.org/cl/4641082