[llvm-readobj] Accept -S as an alias for --sections

In GNU readelf, the short option for --sections is upper-case -S.

Note that GNU uses lower-case -s to mean --symbols, while LLVM
uses -s to mean --sections and -t to mean --symbols (-t has yet a
different meaning in GNU). So command-line uses with -S can now
be compatible, but uses with -s or -t are still incompatible.

Patch by Roland McGrath

Differential Revision: https://reviews.llvm.org/D33872

llvm-svn: 308392