allow parsing of old cstate names

[brayniac]

Problem

Older cstate names appended the microarchitecture abbreviation. For
example C1E-SKX for Intel Skylake's C1E state. Since the cstate
parsing logic assumed newer kernel versions which use a simpler
naming convention, it does not properly track time in each cstate
on older kernels.

Reported in #224

Solution

Improves the cstate parsing to split on '-' and take the first
token as the cstate name.

Result

As a result, the cstate sampling should work on older kernels.