perf db-export: Also export thread's current comm

perf db-export: Also export thread's current comm

Currently, the initial comm of the main thread is exported. Export also
a thread's current comm. That better supports the tracing of
multi-threaded applications that set different comms for different
threads to make it easier to distinguish them.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20190710085810.1650-13-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>