ext: Link gem5 libelf to ext/libelf/libelf.a

ext: Link gem5 libelf to ext/libelf/libelf.a

Currently, gem5 might use system's libelf library instead of
the one compiled from ext/libelf.
This commit tells scons to use ext/libelf version.

JIRA: https://gem5.atlassian.net/browse/GEM5-756

Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Change-Id: I8dc4555c32a956e9f5249288c71982fa6a3678f7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33941
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>