perf unwind: Suppress massive unsupported target platform errors

When cross-analyzing perf data recorded on an another platform, massive
unsupported target platform errors are printed. So let's show this message
as warning and only once.
Signed-off-by: Changbin Du <changbin.du@huawei.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Hui Wang <hw.huiwang@huawei.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230426032246.3608596-1-changbin.du@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/unwind-libunwind.c

@@ -56,7 +56,7 @@ int unwind__prepare_access(struct maps *maps, struct map *map, bool *initialized
 	}
 
 	if (!ops) {
-		pr_err("unwind: target platform=%s is not supported\n", arch);
+		pr_warning_once("unwind: target platform=%s is not supported\n", arch);
 		return 0;
 	}
 out_register: