Fix rpath for host benchmarks

Host benchmarks are installed in
out/host/*/nativetest/<module>/<benchmark> now, so the rpath needs to
include ../../lib.  Make benchmarkLinker extend testBinaryLinker in
order to pick up the correct rpath.

Change-Id: I3c34aee8d3c21a08b75258c1d5caf6c9fb6feab3