[Flang][Driver][NFC] Improve -emit-obj unit tests

Instead of looking for the existence of a `ret` instruction after disassembly
as a way to check that the right ISA is present in the produced binary, check
instead the ELF header of the file. This is a more reliable solution, since the
alternative can easily result in false positives.

Differential Revision: https://reviews.llvm.org/D149578