perf evsel: Reduce a variables scope

In __evsel__config_callchain avoid computing arch until code path that
uses it.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Dominique Martinet <asmadeus@codewreck.org>
Cc: Ze Gao <zegao2021@gmail.com>
Cc: Yang Jihong <yangjihong1@huawei.com>
Cc: Weilin Wang <weilin.wang@intel.com>
Link: https://lore.kernel.org/r/20240918223116.127386-1-irogers@google.com


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