perf trace: Introduce accessors to trace specific evsel->priv

perf trace: Introduce accessors to trace specific evsel->priv

We're using evsel->priv in syscalls:sys_{enter,exit}_SYSCALL and in
raw_syscalls:sys_{enter,exit} to cache the offset of the common fields,
the multiplexor id/syscall_id in the sys_enter case and syscall_id + ret
for sys_exit.

And for the rest of the tracepoints we use it to have a syscall_arg_fmt
array to have scnprintf/strtoul for tracepoint args.

So we better clearly mark them with accessors so that we can move to
having a 'struct evsel_trace' struct for all 'perf trace' specific
evsel->priv usage.

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-dcoyxfslg7atz821tz9aupjh@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>