[lld][RISCV] Use an e_flags of 0 if there are only binary input files.

Summary:
If none of the input files are ELF object files (for example, when
generating an object file from a single binary input file via "-b
binary"), use a fallback value for the ELF header flags instead of
crashing with an assertion failure.

Reviewers: MaskRay, ruiu, espindola

Reviewed By: MaskRay, ruiu

Subscribers: kevans, grimar, emaste, arichardson, asb, rbar,
johnrusso, simoncook, sabuasal, niosHD, kito-cheng, shiva0217,
zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o,
rkruppe, PkmX, jocewei, psnobl, benna, Jim, lenary, s.egerton,
pzheng, sameer.abuasal, apazos, luismarques, llvm-commits, jrtc27

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71101