perf trace beauty: Allow accessing syscall args values in a syscall arg formatter

perf trace beauty: Allow accessing syscall args values in a syscall arg formatter

For instance, fcntl's upcoming 'arg' formatter needs to look at the
'cmd' value to decide how to format its value, sometimes it is a file
flags, sometimes an fd, a pointer to a structure, etc.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-2tw2jfaqm48dtw8a4addghze@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>