K8 is the ancient/initial 64-bit AMD processers. We are retaining
support for these, but our AMD hwpmc class has grown substantially. We
don't want to emphasize this old naming as it is a potential source of
confusion.

The PMC_CLASS_K8 name is retained as an alias, and still used in the K8
code in libpmc.c.

Test Plan

Notably, this changes the output of pmccontrol -l:

24 AMD_ZEN CPUs present, with 39 PMCs per CPU
#CPU 0:
#N  NAME             CLASS  STATE    ROW-DISP
 0  SOFT0            SOFT   ENABLED  FREE      
 1  SOFT1            SOFT   ENABLED  FREE      
 2  SOFT2            SOFT   ENABLED  FREE      
 3  SOFT3            SOFT   ENABLED  FREE      
 4  SOFT4            SOFT   ENABLED  FREE      
 5  SOFT5            SOFT   ENABLED  FREE      
 6  SOFT6            SOFT   ENABLED  FREE      
 7  SOFT7            SOFT   ENABLED  FREE      
 8  SOFT8            SOFT   ENABLED  FREE      
 9  SOFT9            SOFT   ENABLED  FREE      
 10 SOFT10           SOFT   ENABLED  FREE      
 11 SOFT11           SOFT   ENABLED  FREE      
 12 SOFT12           SOFT   ENABLED  FREE      
 13 SOFT13           SOFT   ENABLED  FREE      
 14 SOFT14           SOFT   ENABLED  FREE      
 15 SOFT15           SOFT   ENABLED  FREE      
 16 TSC              TSC    ENABLED  FREE      
 17 AMD-0            AMD    ENABLED  FREE      
 18 AMD-1            AMD    ENABLED  FREE      
 19 AMD-2            AMD    ENABLED  FREE      
 20 AMD-3            AMD    ENABLED  FREE      
 21 AMD-4            AMD    ENABLED  FREE      
 22 AMD-5            AMD    ENABLED  FREE      
 23 AMD-L3-0         AMD    ENABLED  FREE      
 24 AMD-L3-1         AMD    ENABLED  FREE      
 25 AMD-L3-2         AMD    ENABLED  FREE      
 26 AMD-L3-3         AMD    ENABLED  FREE      
 27 AMD-L3-4         AMD    ENABLED  FREE      
 28 AMD-L3-5         AMD    ENABLED  FREE      
 29 AMD-DF-0         AMD    ENABLED  FREE      
 30 AMD-DF-1         AMD    ENABLED  FREE      
 31 AMD-DF-2         AMD    ENABLED  FREE      
 32 AMD-DF-3         AMD    ENABLED  FREE      
 33 IBS-FETCH        IBS    ENABLED  FREE      
 34 IBS-OP           IBS    ENABLED  FREE      
 35 MPERF            PERF   ENABLED  FREE      
 36 APERF            PERF   ENABLED  FREE      
 37 RAPL_ENERGY_PKG  RAPL   ENABLED  FREE      
 38 RAPL_ENERGY_CORES RAPL   ENABLED  FREE      
#CPU 1:
...