Use targetPrefix for bintools binaries when running tests

[r-burns]

Describe the bug

When cross-compiling to a host platform whose binaries are executable by the build system (e.g. x86_64-linux-gnu -> x86_64-linux-musl cross), it is typically possible to also run that packages' tests. This cross-compilation behavior has recently been enabled in nixpkgs.

patchelf's tests refer to unprefixed bintools binaries on the path, which does not work in this scenario, since in cross-compiling scenarios these binaries will be prepended with the host's targetPrefix. E.g. for the previous example, we need to use x86_64-unknown-linux-musl-readelf instead of plain readelf.

Steps To Reproduce

nix-build '<nixpkgs>' -A pkgsCross.musl64.patchelf

An example failure log can be viewed here: https://hydra.nixos.org/build/194616146/nixlog/2

Expected behavior

patchelf should either detect the targetPrefix from the toolchain, or allow the user to specify the targetPrefix at configure time.

Here is a postPatch for nixpkgs' patchelf derivation which currently fixes this, for patchelf 0.15:

  postPatch = ''
    substituteInPlace tests/add-debug-tag.sh --replace \
      '$(readelf' '$(${stdenv.cc.targetPrefix}readelf'
    substituteInPlace tests/no-rpath-pie-powerpc.sh --replace \
      '$(readelf' '$(${stdenv.cc.targetPrefix}readelf'
    substituteInPlace tests/force-rpath.sh --replace \
      '$(objdump' '$(${stdenv.cc.targetPrefix}objdump'
    substituteInPlace tests/phdr-corruption.sh --replace \
      '$(readelf' '$(${stdenv.cc.targetPrefix}readelf'
    substituteInPlace tests/no-gnu-hash.sh --replace \
      strip ${stdenv.cc.targetPrefix}strip
  '';

However, it is probably more robust to address this upstream.

[r-burns]

Nice fix, thanks!