[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@363683
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