runtime: refactor Syscall benchmark And add a benchmark where #goroutines>GOMAXPROCS, because it's the most interesting case. Current results on darwin/amd64, Intel Core 2 Duo 2.13 GHz, 2 cores: BenchmarkSyscall	100000000	        56.0 ns/op BenchmarkSyscall-2	50000000	        57.2 ns/op BenchmarkSyscallWork	10000000	       635 ns/op BenchmarkSyscallWork-2	10000000	       315 ns/op BenchmarkSyscallExcess	 1000000	      2698 ns/op BenchmarkSyscallExcess-2	 5000000	      1192 ns/op BenchmarkSyscallExcessWork	 1000000	      2832 ns/op BenchmarkSyscallExcessWork-2	 2000000	      1966 ns/op
runtime: refactor Syscall benchmark
And add a benchmark where #goroutines>GOMAXPROCS,
because it's the most interesting case.
Current results on darwin/amd64, Intel Core 2 Duo 2.13 GHz, 2 cores:
BenchmarkSyscall	100000000	        56.0 ns/op
BenchmarkSyscall-2	50000000	        57.2 ns/op
BenchmarkSyscallWork	10000000	       635 ns/op
BenchmarkSyscallWork-2	10000000	       315 ns/op
BenchmarkSyscallExcess	 1000000	      2698 ns/op
BenchmarkSyscallExcess-2	 5000000	      1192 ns/op
BenchmarkSyscallExcessWork	 1000000	      2832 ns/op
BenchmarkSyscallExcessWork-2	 2000000	      1966 ns/op

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7415044

apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,gnanderson/go,tv42/old-go,tv42/old-go,apparentlymart/golang,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,dsriram/golang,dsriram/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme