sync: replace Mutex benchmarks

sync: replace Mutex benchmarks
For both contended and uncontended case:
 - support arbitrary number of cpus (not just 2)
 - dynamic load balancing (improves stability)
 - periodic execution of Gosched() to work around non-preemptiviness
For uncontended case eliminates possible false-sharing.
For contended case includes additional variation with some
amount of local work between mutex operations.

R=r, rsc
CC=golang-dev
https://golang.org/cl/4634093