perf trace: Generalize the syscall_fmt find routines

To allow them to be used with other stuff, such as tracepoints.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-od3gzg77ppqgnnrxqv40fvgx@git.kernel.org


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