perf trace: We need to consider "nr" if "__syscall_nr" is not there

To cope with older kernels that don't have this patch backported:

  026842d1 ("tracing/syscalls: Rename "/format" tracepoint field name "nr" to "__syscall_nr:")

This makes 'perf trace' work again in RHEL7 kernels.

Cc: Taeung Song <treeze.taeung@gmail.com>
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-6h1syw2isegnhb1bjmtr9x9k@git.kernel.org


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