perf script: Make printing flags reliable

Add a check for the generated string of flags.  Print out the raw number
if the string generation fails.

Use the SAMPLE_FLAGS_STR_ALIGNED_SIZE macro to replace the value '21'.

Reviewed-by: Ian Rogers <irogers@google.com>
Reviewed-by: James Clark <james.clark@linaro.org>
Signed-off-by: Leo Yan <leo.yan@arm.com>
Reviewed-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20250304111240.3378214-2-leo.yan@arm.com


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