perf trace: Introduce syscall__augmented_args() method

perf trace: Introduce syscall__augmented_args() method

That will be used by trace__sys_enter when we start combining the
augmented syscalls:sys_enter_FOO + syscalls:sys_exit_FOO.

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: https://lkml.kernel.org/n/tip-iiseo3s0qbf9i3rzn8k597bv@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>