[flang] Make tests depend on llvm-readobj not llvm-readelf

d4320cb2 fixed a regular build
issue by making the flang tests depend on llvm-readelf.

Turns out llvm-readelf is a symlink to llvm-readobj and for
whatever reason, doesn't have the right exports at the moment
to be picked up in a standalone build.

https://lab.llvm.org/buildbot/#/builders/175/builds/26448

Maybe we should fix that, but for now, just require llvm-readobj
instead.