[flang] Return 1 in ERROR STOP without user provided stop-code (#87501)

See F'2023 section 11.4: "If the stop-code in an ERROR STOP statement is
of type character or does not appear, it is recommended that a
processor-dependent nonzero value be supplied as the process exit
status"

Fixes https://github.com/llvm/llvm-project/issues/66581.