perf: unwind: Fix symfs with libdw

Pass the full path including the symfs (if any) to libdw.  Without this
unwinding fails with errors like this when a symfs is used:

  unwind: failed with 'No such file or directory'"

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: kernel@axis.com
Cc: Ian Rogers <irogers@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Link: https://lore.kernel.org/r/20230630-perf-libdw-symfs-v2-1-469760dd4d5b@axis.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>