perf trace: Hide evsel->access further, simplify code

Next step will be to have a 'struct evsel_trace' to allow for handling
the syscalls tracepoints via the strace-like code while reusing parts of
that code with the other tracepoints, where we don't have things like
the 'syscall_nr' or 'ret' ((raw_)?syscalls:sys_{enter,exit}(_SYSCALL)?)
args that we want to cache offsets and have been using evsel->priv for
that, while for the other tracepoints we'll have just an array of
'struct syscall_arg_fmt' (i.e. ->scnprint() for number->string and
->strtoul() string->number conversions and other state those functions
need).

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-fre21jbyoqxmmquxcho7oa0x@git.kernel.org


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