perf trace: Move some scnprintf methods from syscall to syscall_arg_fmt

Since all they operate on is on a syscall_arg_fmt instance, so move them
to allow use it from the upcoming tracepoint fprintf routine.

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-ynttrs1l75f0x9tk67spd7jd@git.kernel.org


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