perf tools: Cache counter names for raw samples on s390

perf tools: Cache counter names for raw samples on s390

Searching all event names is slower now that legacy names are
included. Add a cache to avoid long iterative searches. Note, the
cache isn't cleaned up and is as such a memory leak, however, globally
reachable leaks like this aren't treated as leaks by leak sanitizer.

Reported-by: Thomas Richter <tmricht@linux.ibm.com>
Closes: https://lore.kernel.org/linux-perf-users/09943f4f-516c-4b93-877c-e4a64ed61d38@linux.ibm.com/
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>