use readelf from cross-build toolchain in hardfloat test

it's a bit hacky that readelf is hardcoded in the first place (instead
of coming from the spec), but whatever.

Task-number: QTBUG-30677
Change-Id: Id3742c4a47476c8807eff3f566e45fa6d4b97326
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>