ext: Revert "ext: remove libelf"

This reverts commit fa13042b5a1b4120bb7d96d15170bbd5d5068fad,
which removes ext/libelf.

Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Change-Id: Id0ffb480fa5f5fe8faa4816d367b580ebe4c38d7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33939
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
59 files changed