GCC doesn't find libgcc_s.so anymore and looks in the wrong
location.  Fix this (well, more a work-around really).

* gnu/packages/cross-base.scm
  (cross-gcc-arguments)<#:configure-flags>:
  Add --with-toolexecdir, such that libstdc++ ends up in the right
  place. Add --with-slibdir such that libgcc_s.so end up in the right place.
  (cross-gcc-arguments)<#:phases>{move-shared-libraries}:
  New phase, moving libraries in the correct place.
  Delete .la files, libasan.so and libusan.so to prevent circular
  references. Likewise, fix a reference in libstdc++.so.VERSION-gdb.py.
---
 gnu/packages/cross-base.scm | 63 +++++++++++++++++++++++++++++++++++--
 1 file changed, 60 insertions(+), 3 deletions(-)