Uses RDTSC processor instruction in a tight loop to measure how
much time it takes for CPU to churn a million clock ticks. Then
computes clock rate in GHz from this.
== Build
$ curl -O https://raw.githubusercontent.com/ITikhonov/ghz-osx/master/clockspeed.c
$ make clockspeed
cc clockspeed.c -o clockspeed
== Run
$ ./clockspeed
1.4 GHz