[RFE] broader platform support for hinv.cpu.clock metric

[natoscott]

@christianhorn noticed recently that non-x86_64 platform's /proc/cpuinfo file lacks the information that backs the popular clock rate metric. However, sysfs files below /sys/devices/system/cpu/cpuN/cpufreq/scaling_cur_freq do contain this information for other platforms - and we already have code reading these sysfs files for other metrics - it'd be good to use these as an alternate source for this metric.