For syscalls we need to cache the 'syscall_id' and 'ret' field offsets
but as well have a pointer to the syscall_fmt_arg array for the fields,
so that we can expand strings in filter expressions, so introduce
a 'struct evsel_trace' to have in evsel->priv that allows for that.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.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-hx8ukasuws5sz6rsar73cocv@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>