Perf importer doesn't handle big-little systems

Whenever a perf profile contains samples from multiple kinds of cpu cores (cpu_core and cpu_atom in the case of my system), only samples for the first kind of cpu core will be picked. Given that most programs start on the little cores and then quickly migrate to the big cores, this causes almost all samples to be ignored by speedscope. In my case only the first three samples (750us total) were shown by speedscope, while everything else (20s+) was ignored.

Start by locating the perf importer and reproduce the issue with a profile containing samples from both cpu_core and cpu_atom cores. Trace how the importer selects core types; done means samples from all core kinds are shown by speedscope instead of only the first kind.

Written by the indexing model from the issue text.