perf trace: Factor out the initialization of syscal_arg_fmt->scnprintf

perf trace: Factor out the initialization of syscal_arg_fmt->scnprintf

We set the default scnprint routines for the syscall args based on its
type or on heuristics based on its names, now we'll use this for
tracepoints as well, so move it out of syscall__set_arg_fmts() and into
a routine that receive just an array of syscall_arg_fmt entries + the
tracepoint format fields list.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-xs3x0zzyes06c7scdsjn01ty@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>