[llvm-readobj] Allow --hex-dump/--string-dump to dump multiple sections

1) `-x foo` currently dumps one `foo`. This change makes it dump all `foo`.
2) `-x foo -x foo` currently dumps `foo` twice. This change makes it dump `foo` once.
   In addition, if foo has section index 9, `-x foo -x 9` dumps `foo` once.
3) Give a warning instead of an error if `foo` does not exist.

The new behaviors match GNU readelf.

Also, print a new line as a separator between two section dumps.
GNU readelf uses two lines, but one seems good enough.

Reviewed By: grimar, jhenderson

Differential Revision: https://reviews.llvm.org/D63475

llvm-svn: 363683