perf trace: Allow syscall_arg beautifiers to set a different return formatter

perf trace: Allow syscall_arg beautifiers to set a different return formatter

Things like fcntl will use this to set the right formatter based on its
'cmd' argument.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-4ea3wplb8b4j7aymj0d5uo0h@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>