linux: use /proc/cpuinfo for CPU frequency
linux: use /proc/cpuinfo for CPU frequency

Obtain the CPU frequency from /proc/cpuinfo because there may not be any
cpufreq info available in /sys. This also means that the reported CPU speed
from now on is the *maximum* speed, not the *actual* speed the CPU runs at.

This change only applies to x86 because ARM and MIPS don't report that
information in /proc/cpuinfo.

Fixes #588.

unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso