[lld][NFC] Update tests to use -S instead of -s when using llvm-readelf.

Summary: llvm-readobj/readelf accepts both -s and -S as aliases for --sections. However with GNU readelf only -S means --section, and -s means --symbols. I would like to make llvm-readelf more compatible.

Reviewers: MaskRay, espindola

Reviewed By: MaskRay

Subscribers: emaste, arichardson, steven_wu, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D54118

llvm-svn: 346164