enhance systemtools.py to also support POWER systems

fixes issues reported in https://github.com/hpcugent/easybuild/issues/32 by @chwilk
ok, if we are assuming this is the case, `os.path.exists(max_freq_fp)` should be a conditional (cpufreq or not), and all exceptions be treated as real errors. no sneaky return in `try/except` blocks; the logic should be clear.