perf tools: Introduce event_format__fprintf method

perf tools: Introduce event_format__fprintf method

The existing one, event_format__print() uses stdout unconditionally,
and 'perf trace' needs to use it to format into a file that may have
been set by the user, i.e. 'trace -o file.output'.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/n/tip-7l0mgm91hwg0bby00s5pse8r@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>