gnu: cross-binutils: Build with `--with-sysroot'.

This fixes resolution of DT_NEEDED entries in cross-built libraries.
For instance, if ltdl.so needs libdl.so and has it in its RUNPATH, then
libdl.so is searched for in the right place.

* gnu/packages/cross-base.scm (cross-binutils): Pass
  `--with-sysroot=/no-such-path'.