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://golang.org/cl/
7415044