perf trace: Allow specifying names to syscall arguments formatters

For tracepointless syscalls, like clone, otherwise get them from the
tracepoint's /format file.

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-ml5qvv1w5k96ghwhxpzzsmm3@git.kernel.org


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