perf trace: Allow allocating sc->arg_fmt even without the syscall tracepoint

perf trace: Allow allocating sc->arg_fmt even without the syscall tracepoint

At least "clone" doesn't have (enter, exit) entries tracefs/events/syscalls/,
but we can provide a syscall_fmt and use it instead, as will be done for
"clone" in the next cset.

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-o12kejgcxddyovn2hlg4gbim@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>