perf trace: Use the syscall_fmt formatters without a tracepoint

Previously we only used the syscall_fmt when we had sc->tp_format set,
i.e. when we found the (enter, exit) pair in tracefs/events/syscalls/.

But we really only need to use what is in sc->arg_fmt to apply the arg
beautifiers to the syscall argument values, so do it.

With this we will be able to provide formatters to the "clone" syscall,
which doesn't have entries in tracefs/events/syscalls/.

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-y41nl41jrayjo5ucnde2peix@git.kernel.org


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