perf ftrace: Trace system wide if no target is given

This align ftrace to other perf sub-commands that if no target specified
then we trace all functions.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: http://lore.kernel.org/lkml/20200510150628.16610-2-changbin.du@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>