libpmcstat: parse {a,b,c} brace-list event group syntax

[raghavendra.kt_amd.com]

Summary

Parse the brace-list event syntax "{a,b,c[,...]}" used by pmcstat -b.
The parser splits on commas at the top level of the brace and leaves
every other delimiter (including event qualifier ":" and pmu-event
sub-options) untouched.

For each comma-separated token a pmcstat_ev is built via the new
helper pmcstat_add_one_event(); the leader is marked with
ev_is_leader=1 and all events in the brace share an ev_groupid.
Single events outside any brace fall through unchanged.

Sponsored by: AMD
Signed-off-by: Raghavendra K T <[e-mail]>