perf trace: Allow specifying number of syscall args for tracepointless syscalls

When we don't have syscalls:sys_{enter,exit}_NAME, we had to resort to
dumping all the 6 syscall arguments, fix it by providing that info for
such syscalls, like 'clone'.

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-dfq1jtrxj8dqvqoeqqpr3slu@git.kernel.org


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