[flang] Match the length size in comparison (NFC) (#78302)

The template function call CheckDescriptorEqInt((exitStat.get(), 127) is
deduced to have INT_T equal to std::int32_t instead of std::int64_t, but
the length descriptor points to a 64-byte storage. The comparison does
not work in a big endian.

Co-authored-by: Mark Danial <mark.danial@ibm.com>