[ELF] Remove dependency on hexdump from lit test

hexdump is not part of the GNU coreutils, and so is not required to be able to
build and test LLVM, according to the documentation. This change removes the
dependency on hexdump from a lit test.

Reviewers: grimar

Differential Revision: https://reviews.llvm.org/D36958

llvm-svn: 311335