reduce ABI disruptions when new cpu types and new PMC events are added
in the future.
- Support alternate spellings for PMC events. Derive the canonical
spelling of an event name from its enumeration name in 'enum pmc_event'.
- Provide a way for users to disambiguate between identically named events
supported by multiple classes of PMCs in a CPU.
- Change libpmc's machine-dependent event specifier parsing code to
better support CPUs containing two or more classes of PMC resources.
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