perf db-export: Export synth events

Synthesized events are samples but with architecture-specific data
stored in sample->raw_data. They are identified by attribute type
PERF_TYPE_SYNTH.  Add a function to export them.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20190622093248.581-6-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>