make-bootstrap-tools-cross.nix: Add/fix some .so dependencies

- cloog, ppl, cloogppl aren't used by recent GCCs. Kill references to them.
- Use correct versions of isl, as the current GCC depends
  on non-default versions of them.
- Also clarify isl dynamic libraries are needed in cross
  builds, but not in native builds
- Since aeb3d8ce (bzip2: fix cross build on mingw by using autoconf patch),
  it seems that the bzip2 binary depends on libbz2 when cross compiling.
  So copy libbz2 into the bootstrap tarball as well.
- Curl isn't used in the bootstrap tools since e6f61b4c.