perf unwind: Fix separate debug info files when using elfutils' libdw's unwinder

elfutils needs to be provided main binary and separate debug info file
respectively. Providing separate debug info file instead of the main
binary is not sufficient.

One needs to try both supplied filename and its possible cache by its
build-id depending on the use case.

Signed-off-by: Jan Kratochvil <jan.kratochvil@redhat.com>
Tested-by: Jiri Olsa <jolsa@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>