[llvm-readobj] Update tests because of changes in llvm-readobj behavior

This patch updates tests using llvm-readobj and llvm-readelf, because
soon reading from stdin will be achievable only via a '-' as described
here: https://bugs.llvm.org/show_bug.cgi?id=46400. Patch with changes to
llvm-readobj behavior is here: https://reviews.llvm.org/D83704

Differential Revision: https://reviews.llvm.org/D83912

Reviewed by: jhenderson, MaskRay, grimar